Single Voltage Charging Schemes - The 55 volts method

I am creating a new thread for the further discussion of this concept I have been working on. I am putting it into daily use now on a solar cycled open loop 48v whole house Lifepo4 battery bank so I'll start sharing the results here, if I find anything interesting.

What is the 55v method?

This involves setting all charge parameters to a single voltage. For now, I am using 55v/3.437, it's a round number and it seems like it might be the best compromise. The ideal number could be somewhere else close, 54.9, 55.1v, but it's surely in between 54 and 56v. (Or 27.5v for 24v batteries / 13.75 volts for 12v).

Bulk: 55v
Absorb: 55v
Float: 55v
Balance: Starting at 3.437, and never below 3.437

Why is the 55v method?

A single voltage charging scheme primarily solves two problems:

1. Absorption termination presents a conundrum to a solar system. The charger cannot inherently tell how much of it's output is going to loads vs. battery, so it cannot intelligently terminate the absorption process based on amperage. Either it has to fall back on a timer and just assume that after a set period of time absorption is finished, or communications have to be established between the BMS and the charger so that the BMS can inform the charger when the battery is done absorbing. With a single voltage charging scheme, it does not matter if the charger never exits absorbing, because absorbing at 55v is the same as floating at 55v.

2. Balancing schemes that depend on a high absorption voltage for their balancing pressure are vulnerable to a shortage of balancing activity duration. They are only provided a limited window each day at this absorption voltage in order to get their balancing done. At solar C rates, the absorption window could even be as short as 15 minutes per day. Rather than trying to address this shortcoming with active balancers and ever higher balancing currents in order to squeeze the balancing activity into this window, with a single voltage charging scheme we give the battery balancing pressure for the entire full voltage day (absorb + float duration).

What is wrong with the 55v method?

A single voltage charging scheme primarily causes two problems:

1. At lower bulk and absorption voltages, the maximum charge rate will be reduced. As established by OGG and others, the SOC in the battery at the end of the bulk will be significantly less at a lower voltage, adding to the effect of extending the absorption period. It is possible in some system designs that this limitation could result in the battery bank not accepting the charge as fast as the solar can make it, but I believe at typical solar C rates this is might not be a problem, and whatever charge does not make it into the battery during bulk has the entire rest of the day at the same voltage to get in there. There is no need to rush because the float is not gonna let off the gas.

2. The float charge is naturally going to be higher than is ideal for the chemistry, because of the compromise we are making between it and the bulk/absorb. You would not want to use 3.437 on a continuously floated battery as in a UPS or standby application. I believe that two factors are going to allow the higher float to be acceptable in solar cycled systems: First, since we are bulking and absorbing at a lower voltage than usual, we are not cramming the charge in, and this should leave a bit of vacancy in the chemistry to accept the rest of the day's over voltage. Secondly, our float is naturally limited to a maximum of maybe 6-8 hours due to the daily solar cycle, and we are expecting loads to immediately relieve the pressure at the end of that. This balancing act of undercharging and overcharging may allow the compromise to work without harming the cells. But we wouldn't really know until someone has life cycled a battery this way, so this will probably be the primary ongoing topic of theoretical debate about the method.

The first change I am considering is balancing at 3.438 instead of 3.437. 55v is of course really 3.4375 so 3.437 would lead to the BMS trying to balance all cells at 55v. There's a very fine balance here in the exact calibration of the charger though, I am not seeing my bank holding at a true 55v yet and it's reading more like 54.8, 54.9. This could be the calibration of my solar charge controller.

An interesting note is that in a single voltage charging scheme, these concepts of distinct "modes" or "stages" of charging become obsolete.

A single voltage charge profile can be represented as a single CC/CV power supply curve. This is the same as the supply curve provided by benchtop power supplies, LED drivers, and certain types of fixed voltage AC/DC converters. When we set all stages to the same voltage on a "three stage" solar charge controller, we shape the charge curve into an even square like this.


It's one curve and the battery just moves around it based on the charge it's willing to take, at the start of the day it starts in the constant current region accepting max amps as the voltage rises until it climbs up over the corner into max volts and the amperage tapers. Then, it just slides along the constant voltage region, through the "absorption" and "float" period. The entire charge can be represented as a single mode this way.

The highlighted areas are not exactly relevant to this illustration, that's just Mean Well's representation of where their power supply will fall off the square curve in the red circled area, and the maximum power output in black.

I guess a caveat would be I don't know if solar charge controllers actually form a perfect square like this because of their limited input. They might have more like a flat top and a curved CC region.

I've ran into an immediate problem with my own testing, I underestimated my loads and my production is not going to exceed my consumption even in spring.

So that means I won't be be fully charging regularly at all, as any time spent floating on solar will be lost production and more imports. So my backup charger will kick in at 50% now and solar will only regularly take it to 80 or 90%. I'll do manual full charges once in a while to balance.

On the rate of charging though, I'm not getting any curtailing of production there at 55v though. It's still bulking right up until the 90's and even when it enters absorption it's still taking all the amps the solar can provide up to 97-99 SOC. This is mostly at a very modest .05C though.

hwy17 said:

I'll do manual full charges once in a while to balance.

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No doubt you are watching individual cell voltages.

My best guess is that using the “55” method will result in fewer balance issues as the cells keep up with each other during a relatively more gentle charge cycle.

Thanks for doing the work and publishing your results.

I’m interested in seeing how often you need a full charge rebalance although there won’t likely be a true control group for comparison unless you alternate charge profiles or can run two similar battery banks each with different charge profiles .

JoeHam said:

No doubt you are watching individual cell voltages.

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Yeah I will be. Here's a snippet of some float from the first day, I think they had a decent balance already but had been sitting at mid-SOC for 2 months since the last balance.

Something I'm learning, just in general from running the full system for the first time, is balancing a live pack is a different ballgame, lots of erratic motion going on compared to the crisp convergence you can watch on a pack that's simply being charged with no load on it.



I don't think my charge controller is really pushing them into the 3.438 boundary very hard. It's set to 55 but only seems to get to 54.9. I'm gonna wait and see if I think I want it to push them into it any harder. I'm kind of ok with a wider delta if it doesn't result in a real out balance problem. Runners will still hit it.

On the bulk and absorption side of the equation, I'm not seeing any downside yet.

The batteries take every amp their offered up to 100% charge. This is only at .05-.1C still, but I think that's a fairly common charge rate for whole house solar systems.

Even for those who remain weary of floating at 55, half the conclusion could still be drawn that these greater than 3.437 bulk and absorption voltages might have been defined by the practice of much faster charge rates on smaller batteries, and for big batteries there may not be a valid reason to push over 55v even if you still want to drop back to 54v float.

That’s a pretty noisy graph but it looks like max cell variation is around 20mV ?

Personally, I wouldn’t worry about that as much of the time it’s a smaller delta V.

I’ve been running a hodgepodge of 48V nominal batteries for 5 years now, 300Ah total. My charging and discharging are fairly conservative.

Let me calculate the number of times I’ve worried about rebalancing …………..

Precisely zero. I of course have BMS on each of the four batteries and their passive management has been quite adequate.

JoeHam said:

That’s a pretty noisy graph but it looks like max cell variation is around 20mV ?

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Yeah here's yesterday's graphs. We hit 100% SOC on solar for 15-30 minutes at the end of the day.

It's a 10-25mV delta with them grouping closer to 10 under at the top of the charge spikes and spread out around 20 when in the load dips.

The charge controller set to 55v exactly just isn't driving the pack to 55.0v though, only close to it. It's not pushing them into the balancer very hard at all and only the few that are very slightly running are hitting it. And I actually want to observe this for a while before doing something about it, part of my idea is that if the whole method operates more supportively then it can be gentler and more relaxed as well. Maybe we don't need to chase tiny deltas.

But if that's not true and I get balance problems, then I'll dial the CC voltage up a little bit to get the pack up to 55.0 and push the balancing.

hwy17 said:

But if that's not true and I get balance problems, then I'll dial the CC voltage up a little bit to get the pack up to 55.0

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No way to do the remote sensing thing at the main battery interconnect?

wpns said:

No way to do the remote sensing thing at the main battery interconnect?

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No, that would be nice but I don't think my charge controller has that feature.

All my charge settings are 55.4v and it works for me.

Previously I had boost set to 56.6 and float set to 55.2 . After two hours at boost the inverter would dump power to the grid to bring the voltage down to 55.2. if the grid tied inverter was outputting max and then the inverter was dumping from battery to grid on top of that it could exceed the grid relay current so for me it's best to have all the settings identical.

I looked at my packs yesterday and the range from highest to lowest was 0.001v or .002v

Note. My packs never sit at float. Once the sun goes down the system starts discharging the batteries- if I had a situation where the batteries stayed at float for days on end i would have float be lower than boost

1201 said:

All my charge settings are 55.4v and it works for me.

Previously I had boost set to 56.6 and float set to 55.2 . After two hours at boost the inverter would dump power to the grid to bring the voltage down to 55.2. if the grid tied inverter was outputting max and then the inverter was dumping from battery to grid on top of that it could exceed the grid relay current so for me it's best to have all the settings identical.

I looked at my packs yesterday and the range from highest to lowest was 0.001v or .002v

Note. My packs never sit at float. Once the sun goes down the system starts discharging the batteries- if I had a situation where the batteries stayed at float for days on end i would have float be lower than boost

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My single voltage buddy! Maybe you've been at it a while and I'm joining you. Only a few tens of other highly active forum members remaining for us to get on board. But yes, daily cyclers only.

hwy17 said:

My single voltage buddy! Maybe you've been at it a while and I'm joining you. Only a few tens of other highly active forum members remaining for us to get on board. But yes, daily cyclers only.

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Yeah the float setting is essentially useless for daily cycling so it's really just the boost voltage that matters, and I set mine so I can hopefully get the battery to 100% daily

1201 said:

Yeah the float setting is essentially useless for daily cycling so it's really just the boost voltage that matters, and I set mine so I can hopefully get the battery to 100% daily

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Do you feel that 55v would limit your absorption rate and result in curtailed production?

Just wondering if that's your impression, I'm not gonna take it as a technical argument or claim.

hwy17 said:

Do you feel that 55v would limit your absorption rate and result in curtailed production?

Just wondering if that's your impression, I'm not gonna take it as a technical argument or claim.

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Nah. I export any excess production anyway.

55 would be fine I think as long as the boost voltage is not in the flat part of the curve and I'm pretty sure 55v is outside of the flat part.

It looks like the solar charge controller is actually probably fine on it's 55v setting and calibration. I fully charged with grid and solar simultaneously this morning so that I could let the solar have a go at floating and balancing all afternoon.

I don't really get what happened right at the end here, but there's a distinct 55.0v voltage ceiling before that. Maybe at the end is some kind of faster switching intermittent load and while the higher IR cells are recovering slower from the momentary discharge state the lower IR cells actually take on excess voltage. And then, milliseconds or seconds later, the low performers do finally recover and push the pack voltage high. That's just a rough concept I'm imagining. The alternative is that the charge controllers changed some kind of mode or logic and lost control of it's previously flat 55v ceiling.

If I tilt my head right it sort of looks like the group is squeezing together a bit.

I understood Open Cell to be like the more consistent reading, like it was a steady state reading instead of distorted by the varying loads. But Instantaneous Cell voltages show a lot clearer picture of the balancing threshhold.

This looks like a .007 delta to me, but I'm looking at the peaks of the lowest cell vs peaks of highest cell here. BMS does not agree that it has achieved .007 and is still balancing.