Hi folks,
A PM asked that I share some of the work I'm doing on NiMH batteries but this is a work in progress. I discovered a latent defect in my MRC 989 battery charger that I'm sending back for diagnosis and repair. It had worked perfectly when dealing with 6-cell modules but when I tried to refurbish three modules in parallel, 18-cells but it looks like the 'tickle charge' circuit has become a permanent drain instead of a 'tickle charge source.' Also, I'm sending three refurbished modules to Eric S. who is refurbishing his 2001 Prius battery pack in the vehicle.
For now, treat this link as
"lab notes" and not a final report. I work on the web page as time is available. But now to address the greatest threats to our batteries:
- heat - under high temperatures, the internal pressure increases and the water in the electrolyte seeks an escape route as steam or high temperature gas. As the water vapor leaves, the KOH is left behind but no longer as an electrolyte but small deposits. This reduces the working area of the electrodes causing reduced capacity and increased internal resistance. The mitigation is to park in any available shade to minimize the solar pre-heating. It is no accident that there appears to be more failure incidents in warm climates (the South) and hilly areas (Colorado.)
- heavy charging - charging the batteries is an exothermic reaction, it generates heat in the depth of the batteries. At slow rates and for short durations, this is perfectly fine as the endothermic discharge pretty well balances these local heating effects. But if you "force charge" the battery or descend large hills, say more than 1,000 ft / 300 m., this puts the battery at risk. The mitigation for driving in mountains at high speed is to always descend in "B" so the engine handles the excess, charge energy.
- heavy charging of weak batteries - when the electrolyte gets 'thin' the internal resistance increases so even discharge currents heat the battery. But worse, a small amount of electrolyte is broken down, especially at high rates, into O_{2} and H_{2} gas. I see this during battery reconditioning but with a little heat, these high pressure gasses will seek and find any escape route.
I am experimenting with battery refurbishment, trying to develop ways that "do it yourself" folks, those who feel comfortable rebuilding an engine or other major work, can follow to reclaim NHW11 batteries. I find this a fascinating study apart from my recent problem with the MRC 989.
The more recent NHW20 batteries have much improved housings and terminal lugs and should last substantially longer than the NHW11 batteries. So this suggests replacing the NHW11 modules with NHW20 modules would substantially improve the older, 2001-03 Prius. IMHO, one refurbishment of NHW11 batteries and attention to long-life practices is the way to go.
The ultimate "end of life" is a subjective judgment. Being someone who 'drives until the wheels fall off,' my criteria is having problems in two or more major subsystem:
- engine
- transmission
- steering and suspension
- interior
If only one subsystem has a problem, the vehicle can be repaired and kept in service. But when more than one subsystem has a significant failure, it looks like "the wheels fell off" and call the salvage yard.
Bob Wilson
DRAFT - battery work in progress
