The Neato NiMh charging procedure is explored in detail earlier in this long thread, complete with description from a Neato Robotics representative. There are two NiMh procedures in practice. Common single cell (usually four at once), open-to-the-air chargers, as for AA batteries, use a small drop in charging voltage which occurs when full, when a constant-current charging circuit operates (a standard type of voltage regulator, either linear components or a buck voltage converter or switching power supply). Before this voltage blip the NIMh cells will dissipate heat near full, and with enclosed battery packs the heating is excessive, so the charging is stopped on the basis of a temperature rise to get ahead of over-heating. So these packs contain thermistor sensors, with the Neato ones falling in resistance as the temperature rises. The temperature read is reported over USB and can be observed continuously with the Neato Control program for PC (other thread). Details of these thermistors, tables etc., are earlier in this thread.
The algorithm used is not a temperature level but the rate of increase: charging terminates when the temperature rises faster than 1C/minute -- apparently a standard in NIMh equipment. So a simulating circuit need only impose a sudden step increase in temperature in order to trip this detection circuit. There is a temperature limit I think 60C where the Neato system declares an emergency shutdown, for possible fire hazard I suppose; there are also temperature tripped fuses in the NiMh battery packs. To simulate this step increase it is necessary only to parallel shunt the thermistor with a transistor to lower the pair resistance, which is in series with a fixed resistor supplying an analog voltage input to the cpu -- this voltage simply needs to be lowered. A voltage-to-temperature adapter circuit is shown earlier in this thread where the ICL7665 voltage sensor with internal voltage standard for convenience was found useful. A German hobbyist making lithium replacements for the XV brought that part to my attention. Some ultra-high precision 0.1 per cent resistors may be needed to calibrate this precisely.
The earlier method, used in Germany, was to use the heating of components in balancer circuits, as the voltage rises, to thermally simulate the NiMh heating and thermally couple to the thermistors.
Incidentally the best source of the particular thermistors Neato uses -- there are many types with different characteristics -- is salvaging from discarded Neato NiMh batteries. However, we did identify a standard part which appears closest to the Neato ones, earlier in this thread. One could just dispense with them and use a fixed resistor in principle, abandoning emergency temp detection.
Commercial lithium replacements all use a direct voltage-to-temp simulator while DIY Neato owners used balancing circuits. The problem there was that newer balancing circuits became more efficient and reduced their own heat dissipation, so it became hard to find any ready made balancers suitable. The Botvac
LiFePo4 project shows one that appears to work. Such circuits are specific to the type of cell chemistry, LiFePo4 different from Lithium Ion etc., as it is all voltage dependent and the cell types differ in that.
With direct voltage-to-temperature signaling, care must be taken to coordinate the trip voltage with that of any over-voltage protection circuits added so as not to confuse them.
A complication in NiMh charging is additional phases after initial termination: a "topping off" procedure is used with charging at a lower current, only 1/2 amp, for fixed time period instead of temperature controlled. This phase can proceed in Neato's after the indicator light turns solid green indicating full. The NiMh heating can terminate the charging before the full capacity of the battery is reached.
These follow on phases present requirements for balancer type adapters and so on, which must handle the current involved and what not. Apparently signaling temperature rise again can terminate these phases.
Without empirical proof I assume there is a risk in powering the 12v Botvac
system with a four series lithium ion pack going to 16.8v max, possibly damaging some components. Expert engineers would have to opine.
I seem to recall what Lithium Power did was use a lower voltage three series with a large capacity to compensate, so that the capacity at the upper end of the discharge curve is sufficient for the Botvac
, while the bulk of the capacity at lower voltages is never used. This strategy vs LiFePo4 better voltage fits is one disadvantage of this product, with a sort of misleading capacity spec. -- and higher than necessary cost, for all that unused capacity.
The Neato system has step-down voltage regulators for a 5v supply used for sensors and lidar, a common logic component supply, as well as a 3v lower voltage used for cpu components (as they have gotten lower power over years). These may be linear voltage regulator components, and if supplied with too high an input voltage could over heat -- they are basically forming a resistor network and dissipating heat in the reducing resistance. Again engineering expertise -- or empirical results -- are needed to know exactly what will happen. Of course these low voltage supplies do not support high current drains like the motors. The components on the system board have not been identified so the specs are not known.
Neato Charging (and standard procedure) is a constant current variable voltage regulator, standard type of circuit, supplying a higher voltage than the unconnected cell to push in the charge, increasing the battery terminal voltage as the cells fill. This regulator is on the Neato system board and may be a buck voltage converter instead of linear component (typical for the high current used, 2amps in all Neato models -- battery cell replacements must accommodate this charging rate, a separate spec from discharge capacity). Lithium cells charge to a specified maximum voltage and must be protected from applying higher voltages. Additional charge is sometimes pushed into lithium cells with a constant voltage phase, ending when current falls to zero. Neato does not use this 2nd lithium charging phase, and depending on the cell type this phase may only add another 10 per cent of charge.
As to the power distribution circuit, this was traced for the XV by a senior engineer posting here, but not for the Botvac
. Basically some Mosfet transistor switches and diodes arrange the power path according to system state, charging or running etc., under cpu control. The system is powered by the dock when charging. A problem with a diode in this circuit, D14 in XV models, causes a leak of voltage back into the charging contacts from the battery when running. The system detects connection to the dock from this contact voltage, so the software gets confused and stops running because thinks it is in charging mode (other thread on this). The diode appears to have been under-specified for load and larger capacity replacements cure the problem.