# Charging a big battery with small solar panels very very slowly

I have a 6V, .7amp solar panel that typically puts out 2-5V under normal conditions.

I would like to charge a 12V 5 amp hr lead acid battery over a few days to weeks.

The device that I want to power is a lock that needs 12v at about .8 amps for a fraction of a second. The lock is used only once or twice a week. There is also a 3-5V microcontroller that needs to run for maybe 5min. to prime the lock.

Space and weight aren't an issue so I thought lead acid would be simpler.

Now I'm wondering if it'd make more sense to have a 3.7V lipo battery, use a charging board designed for solar and then boost it to open the lock?

When I looked up the charging info for this battery it says to charge it at 14v. It also says that lower voltages may be needed to trickle charge it if it's totally dead (which I don't see happening if this is set up correctly)

What I can't tell is if I can trickle charge it when it's at about 11.85-11.95V. I suspect that if I try to boost my sad little 6V solar panel to 14V the amperage will be so low what will be the point?

Furthermore, when I tried an out-of-the-box adjustable boost board with the solar panel it only turned on at all in direct sunlight. I need to capitalize on the indirect light too.

Are there any application examples of smaller, lower voltage solar panels being used over a long period of time to slowly build up a larger battery? Or do the loss factors make this unlikely?

• "The lock is used only once or twice a week." what is the minimum time that could occur between lock operations? Jan 20, 2020 at 4:54
• min would be about 3 min. Jan 20, 2020 at 12:39
• 12 x Alkaline C cells with a voltage regulator will power this for about 10 years (the shelf life of the batteries.) || 10 years x 50 weeks x 2 weekly x 0.8A / 1/3600 hour ~<= 0.05 Ah over 10 years. Use reputable band cells. With a boost regulator 4 x C or just maybe D Alkaline cells with a boost converter will suffice. 4 cells give you 4V dead flat = OK for uC abd lock. May 24, 2020 at 4:33
• Your solar panel specs "do not compute:. A 6Vmp PV panel makes 6V+ in more than say 10% of sun (or less). If it makes 2-5V unloaded it's a maybe 4V panel. Boosting to suit will work as long as the boost converter can handle the voltage. Lead acid battery needs >= 12.6V to stop it sulphating. 14V is topping charge in deep discharge use and is useful but not essential. IF you han get 0.7A in full sun at 4V then that will boost to say 13V at 0.7A x 4/13 x 80% say ~= 150 mA. THat is 0.15/5Ah ~= C/30 rate for SLA. That should keep it topped up if you can average say 1 sun hour/day overall. ... May 24, 2020 at 4:40
• Battery will never achieve topping charge at such low charge rate so some very simple Vmax limiting (or maybe even none) will suffice. May 24, 2020 at 4:40

You could easily power the lock without using a solar panel with a maybe 10 year battery lifetime.
12 x Alkaline C cells with a voltage regulator (powered only when action needed) will power this for about 10 years (the shelf life of the batteries.) Less batteries needed if lock will operate on under 12V.

• 10 years x 50 weeks x 2 weekly x 0.8A / 1/3600 hour ~<= 0.05 Ah over 10 years.

• Use reputable band cells. With a boost regulator 4 x C or just maybe D Alkaline cells with a boost converter will suffice.

• 4 cells give you 4V dead flat = OK for uC and lock.

Your solar panel specs "do not compute:.
A 6Vmp PV panel makes 6V+ in more than say 10% of sun (or less).
If it makes 2-5V unloaded it's a maybe 4V panel.
Boosting to suit will work as long as the boost converter can handle the voltage.
Lead acid battery needs >= 12.6V to stop it sulphating.
14V is topping charge in deep discharge use and is useful but not essential.

IF you can get 0.7A in full sun at 4V then that will boost to say
13V at 0.7A x 4/13 x 80% say ~= 150 mA.
That is 0.15/5Ah ~= C/30 rate for SLA.

That should keep it topped up if you can average say 1 sun hour/day overall.
The battery will never achieve topping charge at such low charge rate so some very simple Vmax limiting (or maybe even none) will suffice.

how about a standard boost converter, that uses the battery to absorb the energy of the inductor?

and a low-power comparator to shut off the boost converter once charged to 14 volts

you might call this a flyback converter, with some shutdown.