Algorithm for SmartSolar MPPT to charge LiFePo4

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okami asked Jul 26, '20

Algorithm for SmartSolar MPPT to charge LiFePo4

Hello,

This question is probably not completely new, but since the whole PV topic is still new for me, it's sometimes a hassle to find satisfactory answers.

Some people don't recommend the use of a Victron SmartSolar MPPT with LiFe(Y)Po4 batteries, that have no BMS (especially the Winston WB-LP12V40AH) since they state, that it's mandatory to stop charging after the final voltage level is reached, and that the Victron SmartSolar MPPT wouldn't do that but continue with float charging. Elswhere I read that manually setting the parameters for float charging either to the final voltage level or to 0V would lead to the desired behavior (charging would completely stop, when the desired Voltage level is reached).

While I am planning to buy a LiFePo4 with internal BMS I was still wondering, if stop charging at a certain Voltage limit wouldn't still be necessary to operate the LiFePo4 within a certain SoC window in order to optimize for longer life-span.

I noticed, that there are charge algorythms for LiFePo4 around. I just read, that at least for certain MPPT even default LiFePo4 algorithms exist.

Sounds to me, like the statement cited above is just wrong.

Still I would be glad if someone could give me a conclusive answer on this question.

regards

Okami

mppt charging

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Answer 1 · 2020-07-26T22:30:54Z

nebulight answered · Jul 26, 2020 at 10:30 PM

First off, you should never run any lithium battery without a BMS. I wouldn't listen to anyone's advice on anything when they say don't run a BMS.

Second, when you you need to understand how the charging working (sorry if you already know this). There are bulk, absorption and float charge cycles on almost all chargers. Bulk will put as much current into the battery as it can until it reaches a set voltage. Once it hits that voltage, it will switch to absorption which will keep the voltage stable and the current will reduce until a set amount (each charger is different but you have a ton of control with victron chargers). Finally float will keep the battery at a much lower voltage to keep the battery topped off, but not at too high of a voltage.

I find that the lifepo4 batteries themselves do not require absorption or float. There is very little power to be had by absorption phase and unlike lead acid batteries, you don't need to keep them fully charged so float is also not required. However most drop in lithium batteries BMS will only balance on the top end of the charge curve usually starting at 3.5v per cell (14v). Because of that most balancing will take place during the absorption phase so that's why most lithium settings still have absorption. The lower current of the absorption phase gives the balance function time to do it's job. If you cut off your charger after it hits a certain voltage, the pack will have very little time to balance. As long as the cells stay balanced, this won't hurt anything. However there will most likely be an imbalance of the cells over time which is why almost all lithium batteries have a balance function built into the BMS. If this happens, the BMS will likely disconnect the battery prematurely when the battery approaches full or empty.

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Answer 2 · 2020-07-30T17:32:19Z

okami answered · Jul 30, 2020 at 05:32 PM

Well, I guess you might still be able to balance a LiFePo4 with a special charger with balancing function. I also wonder why Winston should sell this kind of "naked but sealed" 12V blocks, if they had so many disadvantages. I guess you just have to know what you are doing.

"Academic questions" since I bought a different LiFePo4 with BMS. As a "noob" I preferred the additional safety from the protection circuits integrated with the BMS.

What you said, still raises two questions for me: You stated yourself that LiFePo4 would not need absorption or float.

1. Is it possible to stop charging after the bulk phase with the SmartSolarMPPT (as some people say, only MPPT like the ones from Renogy could do this).

2. Did I get it right, that if you limit charging your LiFePo4 to let's say 80% in order to extend its life cycle you still have to charge it to 100% once in a while to give the BMS a chance to do its job balancing the cells of your battery?

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nebulight commented · Jul 30, 2020 at 05:44 PM

1. No, Victron chargers have full three stage charging. However if you enabled advanced settings you can limit the absorption time before it switches to float. Depending on the BMS even the float voltage is high enough for balance to continue.

2. I can't comment on only charging to 80% to extend the life of the battery, but I will say if you don't get above a set voltage every once in a while, your BMS will never hit the "start balance" voltage. That doesn't mean that the cells will go out of balance right away, but over time they might.

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