Long Term Spare HV Battery Storage

Ok battery gurus, I need help. I have a spare HV battery which I am in the process of balancing. Need to store it indefinitely- until one of the 3 aging prii in the family throws the red triangle and goes limp. I read all sorts of stuff on storage of Nimh batteries. Some say do it 80% charged and periodically cycle, some say fully discharged and periodically cycle. Some say 40% SOC and periodically cycle. Some say freeze them while others say don’t. Everybody says cycle. I don’t want to cycle more often than absolutely necessary. I’d just about as soon swap my packs out periodically as do a cycle. Swap out obviously is more labor but a whole lot quicker. My schedule does not facilitate baby sitting the battery chargers.

Here is what I propose and do feel free to shot holes in it. That is the reason I am putting it out.

Proposed plan. After battery balance/reshuffle is complete, levelize the modules by standard method of parallel wiring of the modules. Keep it in a cool but not freezing place - as in basement.

This is the potentially controversial part. Connect a regulated power supply to the entire pack with modules connected in parallel. Set the supply and let it maintain a 7.6 or so voltage on the entire pack. I am thinking that this will counteract the self discharge. No battery cycling planned.

Is this a bad idea?

 

Uart and USN -thanks for the info.

The reasoning for keeping the modules in parallel is first to levelize voltage/ SOC and second to facilitate the float charge. Use one regulated power supply (which is all I have) for the entire pack. However I am also leery of keeping them on float. From what I can tell from scattered online info is that self discharge is the enemy or at least one of them. Thought was to counteract it with a float but it may actually be detrimental. Don't know and have not been able to figure it out. There is some info on keeping NIMH batteries charged in standby for emergency lights and such. As I recollect the suggestion was to pulse charge instead of floating. In reality NiMh is not a good choice for those sorts of applications due to the relatively high self dc and also due to the need for smart charging. And also maybe the need for cycling them periodically. My thoughts for keeping them at a relatively low voltage is that I suspect that the self discharge rate drops with lower SOC. I actually don't know that this is the case and it very well may not be.

Was hoping that someone had actually tried this and would weight in.

A second question. How to determine SOC? I am guessing that 7.8 is around 80% with 7.5 being around 40. This is a guess and only a guess.

 

OK Uart- good info.
SOC by capacity. Being as I am in the process of balancing I know the capacities of each module. Average is about 5400 mahrs on discharge down to 6v following 3 rounds of cycling. I think that dc capacity is the one that really matters. So if I am shooting for a 80% SOC on final charge I go capacity X 0.8 x charging efficiency. whatever that be. Read somewhere that efficiency is near 100% up to around 70% and then it gets worse. So I can see that SOC is not readily determined at least not to great accuracy. Think I'll probably take the shotgun approach and use voltage like you said.

Will take your advice and ask BritPrius to weigh in.

thanks

 

Thanks to everybody,

Seems to me that NIMH batteries tolerate fairly variable storage conditions based upon the different schemes successfully used.

Revised plan. Abandon the float charge idea. I might try it on a cell that is to weak to reuse as an experiment but it will probably be a long experiment before results are evident. Keep the entire pack in II, Nobody seems to have issue with this and also it should facilitate quickly returning the pack to service when needed as modules will stay levelized. I am also thinking that this will help keep modules with higher self discharge rates from going flat and having a reversed cell. It also will allow one voltage reading instead of 28 to keep an eye on things. Might attempt to keep pack at refrigerator temps but don't have a good plan on how to do that just yet. An assembled pack is too big and heavy for a refrigerator and also condensation might be an issue. With junk laying around think I might be able to put something together that will work not be a major construction project and also keep the pack good and dry. I like the idea of keeping he pack in a relatively low voltage range as that may reduce self discharge and the resulting crystalline growth, I am thinking 7.4 to 7.6 range although this may require more frequent charges. Charges are fairly quick. Discharges are slow as a prius in limp mode, So maybe this will not be to ornorous. Before returning to service discharge the pack to 6v and do a full charge, discharge back to around 7.8 v, levelize modules one last time and install.

Shifting gears a bit. Being as I was in no particular hurry since this is a spare pack. I let it sit for 10 days after removal and measured module voltages again. Plotted out the final vs intial voltages ratio to see if I had any bad actors. 3 had self dc higher clearly higher than the rest. Two of these have in fact turned out to be troublesome, requiring extra cycles to restore capacity. One I will not reuse. Here is the anomalous data, cell number 1 had high self dc but is the strongest cell by capacity of the ones that I have balanced- It restored quickly; Don't think it was a reading error. I checked it several times. I will let them rest after finishing and check self dc again.