Many technical details of the thermistors and their role in charging NiMh batteries is included in the experimental LiFePo4 battery project report thread at
http://www.robotreviews.com/chat/viewtopic.php?p=124772#p124772Operation and construction of a DIY adapter for Lithium batteries is included. Skip through irrelevant parts.
The software terminates charging in NiMh mode (set by unpublished USB command SetConfig BatteryType) when temperature rises faster than 1 degree centigrade per minute, easily simulated by an artificial jump of several degrees. The high side battery pack (with the 15v terminal) thermistor is monitored for charging. With a lithium battery adapted to the NiMh charger, the temperature readings shown over USB will not be meaningful as far as actual temperature goes.
The NiMh chemistry heats up when approaching full charge. Enclosed pack chargers use this temperature to detect full charge as the heating needs to be stopped. Open single cell chargers use a small drop in voltage which occurs when full, as they are open to dissipate the heat. Lithium cells do not heat up and are controlled by reaching a maximum voltage.
On the newer Rev64 XV system boards, the software can be switched to the Lithium charging algorithm and lithium replacements made without adapters to the NiMh charger, but they have been made only DIY. The older Rev113 boards do not respond to the SetConfig command. The software is shared with Vorwerk and configured by setup to match the particular hardware, with Vorwerk using lithium batteries in their VR100 version of XV Neato models.
While it was not practical to make a LiFePo4 battery with the required capacity at the time of the experiments, later model cells were found adequate for converting original
Botvac models to lithium. The
Botvac runs on 12v vs 15v in XV models, reducing the number of cells required. LiFePo4 is more durable than regular lithium, but at the expense of lower capacity in cells. They are used in some robot lawn mowers, with room for enough cells.