Controlling battery charge percentage


New Member
Oct 3, 2021

I was just watching this video from Will Prowse on battery longevity:

Part of his point is that if you keep the batteries at reasonable temperature and charged somewhere between about 55% - 85%, the batteries will last a lot longer.

So I have a question:
Is there a component that can be added to a solar system, or that may already be included in a charge controller, that would help maintaining the battery charge in that range, instead of always charging to 100%?



Photon Sorcerer
Mar 20, 2021
If the charging voltage is reduced you will get less than 100% charge. Match that with sufficient battery capacity to avoid discharging to the bottom. That is my low tech method.


Photon Sorcerer
May 28, 2020
Is there a component that can be added to a solar system, or that may already be included in a charge controller
yes, the target voltage, absorption or boost voltage. However any voltage over 3.4 volts per cell, 13.6 volts for a 12 volt battery will, given time, almost fully charge the cell/battery. A more practical voltage would be 13.8 volts (for a 4 cell battery) to achieve over 99% state of charge where the charge current is around 0.2C, 20 amps per 100 Ah of battery.
If the charger has a float voltage set to the resting voltage of the battery, around 13.4 volts.
When discharging the battery try to keep above 12.0 volts on a 4 cell battery.

Referring to the chart the ideal working range is in the green area.

Charge the battery up with low stress voltages, when full stop charging and use. If the voltage falls below 13 volts consider a recharge.
If you are not using the battery for a while set absorption and float to just below 13.4 volts.

state of charge.jpg


Solar Boondocker
Aug 14, 2020
I've not found voltage to be an easy thing to measure for SOC for lead acid and more so for lithium.

I would think that anything to reliable do what you are asking for and shut off at 80% SOC would need to be programmed from a shunt, the only reliable, easy way to measure SOC. A Victron shunt has options for relays, which you might configure based off SOC, but no easy way to do that.

What I am doing for my lithium is setting my charge voltage to my batteries 24 hour resting voltage after charged, which is close to the 3.4 volts mentioned. I'm sure its not 100% which is 3.65 volts, but with the lithium charge profile being flat most of the time than steep just before full or dying (non-linear), I don't know if SOC is 80%-90%

There's voltage charts for both lithium and Lead Acid batteries, but with both those are resting voltages. There's a difference when my batteries are sitting at a certain voltage during a charge taking in amps when they are low, but at the same voltage when the amperage starts to taper back to nothing. This voltage and amperage seems to change every day based off how depleted my batteries were and weather as my batteries start to charge.

Any time I monitor voltage hoping to see SOC, there's something that throws it off. Aside from charging or floating, Voltage sag is another. The first night out, I set the voltage SOC to 60% and got a message, but found out someone left a 1000 watt cooker on which caused voltage sag.

With an FLA battery, you can do a hydrometer check, but that is not a practical way to get a SOC. Maybe every few weeks to see if it needs to be equalized, but not more than that.

I can't speak too much to the floor at 2.5 volts, but I know the lithium battery will recover volts when resting. I do a .2 C capacity test on my individual cells the capacity tester shuts off at 2.5 volts, but immediately starts to climb, to where several hours later its not 2.5 but 2.85 volts. I'm sure the battery did not magically gain back amp hours. I had just drained 26 amp hours out of a 25 ah cell, so that extra .35 volts did not put extra capacity in.

Edit: The other thing about not floating the lithiums is what do you do to make sure you do not go into the night where you would need to start charging in an hour or two. I have not figured this out and will continue to float. Even if I figure this out, there's still the becoming cloudy when need to charge.