Who's using LIFEPO4 batteries?

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Who's using LIFEPO4 batteries?

Postby AdrianM » May 19th, 2017, 1:57 pm

I'm curious to know how many people are using LIFEPO4's and whether or not they use cell balancing boards.

There's a fair amount of discussion in forums relating to Electric Vehicles where this particular battery chemistry is very popular (mostly because of safety and performance rather than the maximum energy density drone flyers crave!). One topic that is sure to animate the EV crowd is cell balancing. Two schemes are in use: Top Balancing and Bottom Balancing. Flame wars sometimes break out around the topic so let's tread carefully and respectfully :D

The thing is, most people understand the need to balance cells so that during charge/discharge no cells get over or under charged. Less people seem to be aware that with LIFEPO4 the most damage comes from undercharge (i.e. draining cells below their safe minimum terminal voltage). Over-discharge has definitely proven to be the easiest way to wreck cells judging from my own and apparently other people's experience. Manufacturers don't tend to publish such "value judgements" - just hard data - but a specification such as "minimum resting terminal voltage = 2.5V" does nothing to convey quite how important this is compared to, say, taking a cell a little higher than its maximum charging potential. Neither are clever, but one is more expensive in terms of ruined batteries than the other! (Lithium Polymer is a totally different story though!)

For this reason, Bottom Balancing is a scheme that has gained in popularity. Unlike Top Balancing where current is shunted around cells so they all approach the same maximum "full charge" potential (thus limiting the voltage on every cell to the same final terminal voltage) Bottom Balancing done by starting out with all cells down at the same minimum level of charge (call this zero charge). In this way, the charge state of all cells "track" down to the same "zero" level after a full charge/discharge cycle. No cells are over-depleted while stronger cells (still holding charge) are emptied.

The simple fact is that individual cell capacity (AH) is rarely (if ever) exactly the same as is printed on the pack label. A Series string of cells can only be sensibly used as if it had the AH figure of the weakest cell. Bottom balancing makes this much easier to accomplish - with the added benefit of not needing active cell balancing circuitry which seems to be used without much analysis of the actual situation.
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 20th, 2017, 12:13 am

The only product I have heard of equipped with LiFePo4 batteries is the Robomow lawn mower, with plenty of room for the lower energy density cells.

Protection and balancer PCB's are supplied for LiFePo4, with a different cell voltage than regular lithium ion; try Ebay, Aliexpress, and battery distributors online.

A DIY LiFePo4 battery for Neato Botvac supplied with NiMh batteries (12v system) is at
http://www.robotreviews.com/chat/viewtopic.php?f=20&t=19149

There a balancer was used to heat thermistors emulating NiMh behavior for the charger depending on temperature; the balancer is enhanced to increase balancing current sufficiently over the supplied component values.
This project uses a newer model of LiFePo4 cell which has a bit more capacity than older ones.

The heating balancer method was used by the original lithium ion Neato project in Germany, Roboter-Forum; the original balancer components were discontinued and newer ones tend to have too little balancing current for required heating. Extra resistors in parallel with those on board lower the value for more current.

I experimented with LiFePo4 cells for the Neato XV models with higher 15v voltage than Botvac, and was misled by the over stated capacity; it would take 15 cells (3 parallel 5 series) where space is available for only 12 (single series NiMh). The newer cells in the Botvac project might have worked but not then available. (The 12 cells worked briefly then quickly wore out, over loaded.) Neato thread "Lithium Ion Revisited". There a simple circuit was constructed to synthesize artificial thermistor signals corresponding to voltage normally governing lithium charging, similar to the commercial lithium ion replacement batteries (which have a full complement of protections, probably a microcontroller on board). The ICL7665 dual voltage comparator IC for power supply control is a good part for voltage detection, with internal reference; 0.1 per cent resistors can be used for precision set up. This can be combined with a standard protection board or balancer needed with lithium ion compared to less fragile NiMh (LiFePo4, though, is inherently safe like NIMh without the fire hazard of lithium ion -- but with lower energy density).
LiFePo4 is also described as self-balancing, though must be initially balanced when packs are assembled.

The newer Neato XV robots (with Rev64 Binky system boards, lacking the side charging jack) can also be set to Vorwerk software configuration with USB commands to perform lithium charging, based on voltage instead of heat, eliminating heating balancers, but the voltage is set to the higher level of lithium ion 4 series, a little different than 5 series LiFePo4 which would be used -- still might work. The DIY batteries made this way so far have all been regular lithium ion, 8 cells 4 series. Lithium ion can still last three times or more the life of NiMh cells. The super lifetime of LiFePo4 might be more expensive, and the distant future less relevant (by then robot advances might inspire upgrading any robots to new models).

The latest robot versions have all switched to original lithium ion batteries, much improved over the older designs with NiMh batteries needing inconvenient frequent replacement, even though their prices have fallen over time. Development of the LiNMC chemistry safer than the original type used in cell phones and laptops etc. played a role, with power tool packs switching over etc.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 20th, 2017, 6:00 am

Great round-up glnc222, lot's of interesting discussion material!
glnc222 wrote:The only product I have heard of equipped with LiFePo4 batteries is the Robomow lawn mower, with plenty of room for the lower energy density cells.

I guess there's a commercial advantage to them using the lower end of the energy density range offered by the latest LiFePo4's (drat, looks I'm going to have to camel case this designation if I'm going to be taken seriously :lol: )

glnc222 wrote:Protection and balancer PCB's are supplied for LiFePo4, with a different cell voltage than regular lithium ion; try Ebay, Aliexpress, and battery distributors online.

Sure, but those are for top-balancing which I'd like to demonstrate is an unnecessary and potentially counterproductive scheme for healthy cell maintainence.

glnc222 wrote:A DIY LiFePo4 battery for Neato Botvac supplied with NiMh batteries (12v system) is at
http://www.robotreviews.com/chat/viewtopic.php?f=20&t=19149

There a balancer was used to heat thermistors emulating NiMh behavior for the charger depending on temperature; the balancer is enhanced to increase balancing current sufficiently over the supplied component values.
This project uses a newer model of LiFePo4 cell which has a bit more capacity than older ones.

I'm currently using a ZIPPY Flightmax 4200mAh 4S2P 30C LiFePo4 Pack which just fits in a Roomba 5xx battery compartment. Latest measured capacity is actually 4530mAh down to a baseline charge level at 2.75V per cell.

As described on my OP, the cells are initially discharged in a manual process using a temporary connection to the balance port. This "Bottom Balance" process only needs to be done once - as per manufacturers assembly process but it can be repeated if required to compensate for differential ageing or mishandling. However, if the following two criteria are adhered to, in practice it should not be necessary to re-balance:

Having established a zero reference charge, all that remains is to ensure automatic disconnection of load at this low voltage threshold (LVD should be considered an essential requirement anyway) - and an upper voltage threshold to disconnect charge current before the weakest cell over-tops. Some method to limit charge would also be in place already so we're not adding anything - maybe tweaking.

Note that no ongoing cell balancing is required as charge is being added to and removed from cells while their internal impedances remain in their linear regions. This is where I see a lack of attention in this subject: If we consider a NiMh heating up at the end of charge the I2R losses that produce the heat are the product of charge currrent and internal resistance which has climbed out of the healthy linear region it was in while the cell was happily accepting charge. We rarely see this effect (thankfully) in Li-ion because we're too busy fetching the fire blanket - but it can be considered the same.

Similarly, at the fully depleted end of the charge state, internal resistance climbs once again but with so little current flowing by now, the heating effect is negligible. So between these limits internal impedance effectively flatlines and charge can go in and out without affecting the terminal voltage. Of course there is a slope and it varies between all the different battery chemistries - LiFePo4 having one of the flatest that I know of.

glnc222 wrote:The heating balancer method was used by the original lithium ion Neato project in Germany, Roboter-Forum; the original balancer components were discontinued and newer ones tend to have too little balancing current for required heating. Extra resistors in parallel with those on board lower the value for more current.

I guess electricity is cheap enough, but bypassing (shunting) current is a waste of power but it also disregards the underlying nature of what's actually going on in the cells. It also represents issues of quiescent current and potential for catastrophic failure with permanent connections to electronic components being present across the cells.

glnc222 wrote:I experimented with LiFePo4 cells for the Neato XV models with higher 15v voltage than Botvac, and was misled by the over stated capacity; it would take 15 cells (3 parallel 5 series) where space is available for only 12 (single series NiMh). The newer cells in the Botvac project might have worked but not then available. (The 12 cells worked briefly then quickly wore out, over loaded.) Neato thread "Lithium Ion Revisited". There a simple circuit was constructed to synthesize artificial thermistor signals corresponding to voltage normally governing lithium charging, similar to the commercial lithium ion replacement batteries (which have a full complement of protections, probably a microcontroller on board). The ICL7665 dual voltage comparator IC for power supply control is a good part for voltage detection, with internal reference; 0.1 per cent resistors can be used for precision set up. This can be combined with a standard protection board or balancer needed with lithium ion compared to less fragile NiMh (LiFePo4, though, is inherently safe like NIMh without the fire hazard of lithium ion -- but with lower energy density).
LiFePo4 is also described as self-balancing, though must be initially balanced when packs are assembled.

I never heard this before but it may be inspired by the very shallow discharge slope (i.e. shallow change in parasitic impedance). But yes, if manufactureres could be trusted to balance cells at the bottom end we might only ever need LVD and charge termination set to accomodate the weakest cell.

glnc222 wrote:The newer Neato XV robots (with Rev64 Binky system boards, lacking the side charging jack) can also be set to Vorwerk software configuration with USB commands to perform lithium charging, based on voltage instead of heat, eliminating heating balancers, but the voltage is set to the higher level of lithium ion 4 series, a little different than 5 series LiFePo4 which would be used -- still might work. The DIY batteries made this way so far have all been regular lithium ion, 8 cells 4 series. Lithium ion can still last three times or more the life of NiMh cells. The super lifetime of LiFePo4 might be more expensive, and the distant future less relevant (by then robot advances might inspire upgrading any robots to new models).

This sounds like it might be more reasonable. Are we sure active shunt balancing isn't also employed?

glnc222 wrote:The latest robot versions have all switched to original lithium ion batteries, much improved over the older designs with NiMh batteries needing inconvenient frequent replacement, even though their prices have fallen over time. Development of the LiNMC chemistry safer than the original type used in cell phones and laptops etc. played a role, with power tool packs switching over etc.

I suspect we'll see LiFePo4 taking over from Li-ion as manufacturers are forced into considering their liability. I notice supermarkets are now stocking their shelves with cheapo imported solar garden lights fitted with LiFePo4. That's bound to bring prices down!
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 20th, 2017, 1:38 pm

[edit] Zippy LiFePo4 pack mentioned, UK https://hobbyking.com/en_us/zippy-flightmax-4200mah-4s2p-30c-lifepo4-pack.html?___store=en_us

The general purpose balancer used in the German Neato battery project a few years ago was described in an article at the supplier. Single cell board product, stackable in series any length series. It addresses varied rates of wear on cells causing imbalance in packs over long use, shortening pack life unnecessarily. Hobbyists have described repairing packs by disassembling to rebalance etc. Self balancing aspect of LiFePo4 presumably less need for such circuits.

Newer boards have a controller IC for multiple cells on one board, compact for making chargers. They seem made more for chargers with separated cells instead of inclusion in battery packs, i.e. tools for assembling packs initially. Some seem to operate after charging is completed in a separate phase.

Other German DIY Neato projects used synthesized thermistor signals. I have not checked German Roomba battery discussions. Thermistor resistance falls with increasing temperature, so just bypassing with a transistor conveniently simulates heat. NiMh Neato charging software terminates when temperature rises 1C per minute, an industry standard. A single jump suffices.

Separate cell NIMh chargers use a tiny voltage drop when full instead of heat, cells being exposed. A 15 minute fast charging AA type gets so hot I won't use them anymore. Panasonic Eneloop tech now widely imitated removed self-discharge from small NiMh AA cells but not used on high drain, larger cells in robots.

The proprietary adapter boards in Lithium Power Inc. replacement packs for Neato are not observed to heat up in charging I think, so may not perform balancing at least to govern the NiMh charger.. Lithium Power in Taiwan specialized in lithium battery packs for medical equipment many years.

Lithium chargers in robots do not bother with the second, constant voltage phase of lithium charging topping off the main charge with a few more per cent full (only ten per cent on LiFePo4).
Last edited by glnc222 on May 24th, 2017, 7:40 pm, edited 1 time in total.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 20th, 2017, 6:08 pm

This patent assigned to Black & Decker Inc. describes the bottom balancing process in some detail. The methods described include an automated cell discharge phase to bring all the cells down to the same zero reference charge, but in practice this need only be done once with a temporary connection (via standard-fit battery balance port) and can be performed manually with a DVM and dummy load. I use a 21W 12V filament lamp from a car brake light which draws around 1Amp for this purpose.

Once balanced, note that End Of Charge and End Of Discharge points can be determined using only a single voltage measurement across the total of the series string (once the upper and lower knees have been found either by measurement or Delta V) - which can be instrumented using the standard telemetry in most bots. For my Roomba 530 I have an Electric Imp module connected over SCI listening to the voltage data and applying a fake temperature excess (via an open-drain I/O and resistor connected to the thermistor bridge) - or it could be the simplest of Arduino or PIC chips (I used to use an 8-pin PIC12F1822) for the same purpose.

By keeping the same level of charge strictly between lower and upper limits in all cells (which, note, is different to keeping the same voltage across each cell) we maintain a balanced pack with a minimum of additional electronics and intrinsic tendency to avoid the most harmful conditions for this particular battery chemistry.

I probably sound like a salesman, but my actual motive is to test this concept to destruction as I know my knowledge isn't complete and there are many smarter people who's opinions I would appreciate. Other than that, this method does seem to work well for me and I would be pleased if it worked for others contemplating the same issues with LiFePo4 maintenance in their bots. :D
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 20th, 2017, 10:10 pm

To balance cells in packs I discharged them fully using a simple TLC393 comparator controlling a relay for cut off at the minimum voltage; the ICL7665 can be more accurate. Bunches of 10 watt 50 ohm resistors in parallel are an alternative to 12v 25 watt lamps for loads.
[edit]circuit http://www.robotreviews.com/chat/viewtopic.php?p=116932#p116932
Then the assembled pack can be charged from a known discharged state. This was necessary because of accidents discharging cells in DIY packs.

It should also be possible to charge all the individual cells fully in a separate cell charger before assembly, but this instrument may not be on hand or worth the cost.
There are two supposedly known charge positions, fully discharged or fully charged. Useful with new, presumably not worn cells. Analyzing used, partially worn out cells gets complicated and not clearly worthwhile.

An automatic continuous cycler was made to examine long term NiMh battery life under robot usage conditions: http://www.robotreviews.com/chat/viewtopic.php?p=128471#p128471

West Mountain Radio company makes expensive professional equipment for this with fan cooled heavy load modules.
Single cell testers can do similar much less expensively.
Last edited by glnc222 on May 24th, 2017, 7:10 pm, edited 1 time in total.
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 20th, 2017, 10:40 pm

As described on my OP

Apology as amateur not familiar with this term, please supply a link. Also to the patent mentioned if available.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 21st, 2017, 4:53 am

Hi glnc222, OP is short for Opening Post. The Patent in the post above is a hyperlink, but if it's not clickable for you then here it is again: https://www.google.com/patents/US200900 ... mQWknti2BQ
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 21st, 2017, 5:15 am

In case anyone looks at the patent, I ought to point out that I think there are errors in some diagrams. Most relevant to this discussion is figure 7 which shows a flowchart of their Bottom Balance scheme. Here it is with my corrections:
US20090096419A1-20090416-D00004.png


In that chart, the initial discharge phase (up until descision 326) as I've also described it earlier, can be performed manually as an infrequent exercise. So, simply put, balance once then charge and discharge between established low and high thresholds and otherwise leave the pack well alone.

Long term monitoring can be done in software by "keeping an eye" on delta V - too steep during charge or discharge would indicate that capacity has been lost and re-balancing might be necessary. I've implemented this in one bot but it has yet to trigger in the last 6 months or so.
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Re: Who's using LIFEPO4 batteries?

Postby mfortuna » May 21st, 2017, 9:10 am

When NIMH batteries were popular in RC racing we used to do bottom balancing. First discharge the 6 cell pack at 30A until it reaches 5.4V (0.9 per cell). Then put them in a tray so each cell is discharged to 0V. Some racers even soldered a wire across the pack to keep the pack at 0V between races. Then the packs were charged at 5A using delta peak. Never lost a cell and the packs had "punch", the ability to deliver current. These packs started out as matched high voltage, high capacity, low internal resistance cells.

My LIPO balancer for my RC stuff stops charging cell groups when they reach 3.7V. The other groups still charge. One issue with Lithium packs is they are made of cells in series and parallel. There is no way to match the parallel cells. One cell will determine the overall charge.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 21st, 2017, 9:46 am

Hi mfortuna,
mfortuna wrote:When NIMH batteries were popular in RC racing we used to do bottom balancing. First discharge the 6 cell pack at 30A until it reaches 5.4V (0.9 per cell). Then put them in a tray so each cell is discharged to 0V. Some racers even soldered a wire across the pack to keep the pack at 0V between races.

..or just leave them on the shelf for a few days :lol:

Seriously though, that's very interesting. I'm guessing re-charge had to be done almost straight afterwards otherwise the cells would rapidly degrade?

mfortuna wrote:My LIPO balancer for my RC stuff stops charging cell groups when they reach 3.7V. The other groups still charge. One issue with Lithium packs is they are made of cells in series and parallel. There is no way to match the parallel cells. One cell will determine the overall charge.

And having all been topped-off at 3.7V, then on discharge something needs to be done to ensure that the lowest capacity cell isn't taken below its low-voltage threshold right?

I like to think of the situation where you could (if you were very silly) make a series pack out of cells having totally different Ah capacities. Your RC charger would dutifully put say, 4Ah in a 4Ah cell and 1Ah in a 1Ah cell. We better not extract more than 1Ah from this sorry pile! Of course no actual pack with matched Ah cells actually has identical capacities so the same is true in general.

But isn't this also true of bottom balancing? It is in some respects, but having spare capacity unnecessarily remain in some cells subjects them to degradation (at least in applications where our bots need to be kept in readiness) Also, when balanced at the bottom, if whatever conservative margin we've chosen is over-exploited then at least all cells go down in unison rather than causing differential degradation (and at worst, cell polarity reversal)
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 21st, 2017, 7:19 pm

I thought protection boards connect each cell individually in a pack and monitors under and over voltage on each cell, disconnecting the pack when out of range, with mosfet ground circuit switches. They have to be rated, with mosfet capacity, for the pack size and load. In the Neato XV project with Lithium mode, pairs of parallel cells were arranged as two series on separate protection boards then in parallel, every cell on a board separately -- fussy.

Hopefully with longer life of LiFePo4 wear would be less an issue than defects in the pack charger.

Ironically an over voltage condition in the Neato charger constant current mode, caused a system board damage from the charger going to full supply voltage normally not reached, apparently some component could not take. So I put the over-voltage cut off switch on the charger supply instead of on the battery terminal.

It looks like the Black and Decker thing is for their factory pack making instead of user Tool Pack chargers which would lack a load for discharging, off the tool (for swapping packs as needed, similar to camera batteries).
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Re: Who's using LIFEPO4 batteries?

Postby mfortuna » May 23rd, 2017, 8:39 am

A couple comments to AdrianM.

Typically people raced once a week so that is how often the packs were charged. I would charge them the night before and then top them up just before the race. The only effect of this procedure were the cells seemed to lose a bit of capacity but they only had to last 5 minutes in a race.

We paid a premium for these packs. They were made from 3800mah cells that were within 10 mah in capacity and within .05V. I forget the IR values but those were almost identical.

The packs were sold un-assembled but still went for $60-$100 ($10 to $17 a cell !) I bought the $10 cells and did pretty well racing when I was younger.

LIPOs have taken over racing these days.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 23rd, 2017, 1:20 pm

The internal resistance of Lithium batteries is positively scary! My Roombas 4200mAH battery has a C30 rating so discharge of 126Amps has to be expected short-circuit. This no doubt is why a careless moment disconnected my R3's SYS_RET's somewhere inside the multilayer PCB :shock:
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Re: Who's using LIFEPO4 batteries?

Postby a1robotrepair » May 23rd, 2017, 6:36 pm

AdrianM wrote:The internal resistance of Lithium batteries is positively scary!
Indeed. Many under 20mOhm.
My Roombas 4200mAH battery has a C30 rating so discharge of 126Amps has to be expected short-circuit. This no doubt is why a careless moment disconnected my R3's SYS_RET's somewhere inside the multilayer PCB :shock:
Once a resistor has changed due to excessive heat/current they will tend to continue to change, the beta has increased.
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 24th, 2017, 5:55 am

a1robotrepair wrote:Once a resistor has changed due to excessive heat/current they will tend to continue to change, the beta has increased.
Best way to change those Rs: break middle with dikes, use lots of flux and soft solder to remove 4 ends.

Thanks for reminding me to get some replacements - I'm doing a Mouser order today!
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Re: Who's using LIFEPO4 batteries?

Postby glnc222 » May 24th, 2017, 7:46 pm

It would be interesting to see schematics of how you adapted the Zippy LiFePo4 pack to the Roomba 530 NiMh charger -- assuming not proprietary -- no other post found. There is no voltage spec shown by the vendor on that pack.
https://hobbyking.com/en_us/zippy-flightmax-4200mah-4s2p-30c-lifepo4-pack.html?___store=en_us
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Re: Who's using LIFEPO4 batteries?

Postby AdrianM » May 25th, 2017, 5:48 am

The adaptation was mostly just mechanical, I kind of treated it as a challenge because I already had an Electric Imp module installed, and don't subscribe to active charge balancing for LiFePo4 - so set myself the goal of managing the battery charge using the SCI data alone.

In addition to soldering the battery leads directly to the PCB (with an inline 5A polyfuse and bullet connectors) along with the usual SCI wiring, just one other connection was made to the thermistor tab (R52) via a 3K resistor to a GPIO on the Imp to fake a high reading when driven low.

Mehcanically, the battery shrink-wrap and side protection cards all had to be carefully removed (it's a very snug fit indeed) and I drilled out one end of the battery bay to permit the battery leads to exit under the main PCB and into the area behind the bumper. I also de-soldered all four spring contacts for the original battery. I removed the thermistor form the old battery pack and soldered its two leads directly onto the PCB such that the thermistor sits on top of the battery when Roomba is up the right way. This allows me to keep an eye on the LiFePo4 temperature independent from the SCI (via the 3K -> GPIO).

All charge control is then done using the reported battery parameters and faking the high temperature. Any micro-controller would be able to perform the same basic software task of terminating the charge at a given threshold. However I don't thing a simple comparator would do the job because the trickle charge needs to be supressed and that requires sending some additional commands over the SCI. I started another topic about this if you're interested.
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