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Floodie / Lithium hybrid battery experiment

40201 Views 93 Replies 16 Participants Last post by  dougingraham, 
Ziggythewiz
I'm starting an experiment to test the effects of using lithium LiFePO4 cells in parallel with cheapo lead acid batteries.

Lithiums are 40 AH Calbs, and they'll be boosting 29HMs (formerly known as 29DC) and GC8s. My current frankenpack consists of 3 29HMs and 11 GC8s.

Phase 1 of FLHE will put 8 calbs in parallel with 2 29HMs with a JLD404 to monitor AH usage and control charging. The calbs will be charged in parallel with the floodies until approx entering gassing phase where the JLD will cut the lithium contactor while the floodies finish their cycle. 1 29HM will remain unboosted as a control.

Phase 2 will add 8 calbs in parallel with 3 GC8s and tested as above.

The experiment will try to gain info about how the current sharing works out as well as how lead cycle life can be prolonged with the aid of a lithium booster.

A successful experiment would demonstrate an increase in lead cycle life sufficient to justify the cost of the lithium booster.

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TruEnergy
As long as your contactor cuts out at the right voltage, I believe your charge method will work fine. My guess is the lithium will consume the majority of the charge current due to its lower IR.

Ziggythewiz said:
..., and I decided I need some more meters before I'm ready to go.
Click to expand...
Not sure what you have in mind for meters, but I have one of these Doc Wattson Meters, and its pretty neat for the price. It doesn't log, so you have to manually record data at regular intervals. But for tests like this, you'll probably be monitoring it anyway.

http://www.powerwerx.com/digital-meters/doc-wattson-meter-dc-inline.html

I ended up finding one on eBay a while back, for slightly cheaper. If you google it, there seems to be more online stores carrying them now.
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TruEnergy
Nice.

Glad you are able to do this experiment. Thanks, and looking forward to the results.
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TruEnergy
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I wish I had the testing equipment (and an EV!) to do some real world testing like this!!! Good work Ziggythewiz!

For what its worth, I did a simple discharge simulation.

12V Pb 18mΩ
12V LiFePO4 6mΩ
R1 10Ω
R2 1Ω
R3 0.01Ω
Switch 1 'on' at 1s (0.91Ω parallel)
Switch 2 'on' at 3s (0.0099Ω parallel)

At low loads, the two chemistries share current contribution. At higher loads, the LiFePO4 contributes more - limited by its lower IR. I don't have detailed cell models, but this should explain why the Pb voltage sag was less. Being able to log individual pack discharge current and total current should show a similar trend. The question is the rate of power consumption. At relatively "low" loads, they will contribute a similar amount. Under "high" loads the lithium should contribute more. Low and high are very relative because if you never actually reach the "high" part - both chemistries will continue to contribute evenly.

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TruEnergy
Indeed a more detailed battery model would help simulate your findings much more accurately. I have read some university papers where they attempted to create PSpice models of lead batteries. Very difficult and very specific to the brand, internal construction, capacity, and materials used.

It might be possible to create one from something like a PL6 log. If the logs are generated at different discharge rates.

Do you know your total amperage draw? If so, I could program identical pack sizes that you are using and iterate through some load values to get an idea of the current distribution. But since the distribution seems to vary with SOC, you will be faced with finding a "close enough" sharing solution. Let me know if there is anything specific you'd like me to try simulate. I will also continue searching for some battery models.
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