Can somebody comment on sizing a solar cell for trickle charging a lead-acid battery bank without a charge controller for several months?

I found a non-authoritative source that claimed that cells with a power of 4% of the C20 capacity of the bank do not need a charge controller. - This corresponds to a charge current of about 0.23% of C20.

As far as I know, the float charge of lead-acid (if done with constant current) is typically done at 1% of C20 but requires the termination condition of dV/dt = 2.5mV/cell/hr.

So where does this 0.23% of C20 come from? Is the assumption that this balances the self-discharge of the cells perfectly?

Since the self-discharge varies from model to model, how does one size this properly? Size too small and the battery dies from deep discharge, size too large and it dies from slow overcharge?

Also: the balancing of self-discharge with a current of 0.23% of C20 does not sound quite right either. Assuming 5% discharge per month would imply a self-discharge current of the order of 0.007% of C20 - much smaller than the recommended 0.23% of C20 for "solar cell trickle charging".

Can anybody "shed light" on this for me?

  • \$\begingroup\$ Why the vote to close the question? \$\endgroup\$
    – ARF
    Commented May 15, 2013 at 9:54
  • 1
    \$\begingroup\$ THE QUESTION IS ABOUT THE PROPER DESIGN OF A LEAD ACID BATTERY CHARGER. It's a genuine design question with a null set example as the starting point and a question of the validity of the circuit. The question is completely relevant to the general aims of the site. \$\endgroup\$
    – Russell McMahon
    Commented May 15, 2013 at 21:48

2 Answers 2


Battery University is a good starting point. A vast amount here Battery University


Charging Lead Acid

Based on the following, it would SOUND wise to charge fully for "a while" under trickle and then to open circuit the charger until Vbattery fell to approaching 2.1V/cell. A controller to implement this could be extremely simple.

Dangerous in isolation, but, they say:

  • Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.

    Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.

    Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation.

    Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.

  • \$\begingroup\$ Thanks for the link to battery university. It is a very useful ressource. - I take it from your quote that constantly trickle charging is not only "bad" with constant current but also with constant voltage? That is interesting: I knew float charging with constant current requires a termination condition but had always read that a constant voltage float charge of 2.25 to 2.30V/cell could be applied indefinitely. \$\endgroup\$
    – ARF
    Commented May 16, 2013 at 8:01
  • \$\begingroup\$ I found another source on "hysteresis charging": batterytender.com/resources/battery-basics.htm - Apparently "hysteresis charging" is not really float charging but a repeated absorption charge? I am concerned that without voltage regulation a solar cell cannot implement a constant voltage absorption stage. It would result in a repeated boost stage without absorption which would probably not be good for the battery. \$\endgroup\$
    – ARF
    Commented May 16, 2013 at 8:19

Apart from the numbers, which I won't comment on, and the cost of using raw solar cells to float batteries, which is generally poor economics ---

If the Solar Cells, temperature and solar insolation are matched correctly to the Battery Bank, then, at the right time of year, when the Battery Voltage is high, the charge current will be low. Much less than the available power of the Solar Cells. Above the Solar Cell maximum power point voltage, current and power drops off fairly sharply. Correctly matched, the system will be self regulating.

Since the trickle charge turns off when the battery reaches float voltage, it doesn't matter that it starts out as more than the self-discharge value.

To do this, you make the solar cell string short (open circuit voltage at battery voltage) and wide (trickle charge current). This is very inefficient, but it has been economic for toys - and the economics depend on the rapidly changing cost of each element including supervision cost. You can't regulate exactly because of solar variation etc, so you run on the low side and periodically use another charge system, or you discard/replace/repair the battery bank when it over/under charges.


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