Build a variable current limiter for lead-acid battery charging

[Irfan Latif]

I have a 12V, 200Ah deep-cycle lead-acid tubular battery which I charge with solar panels.



PV array specs are:

Max Power                840W
Voltage at Max power     23.70V
Current at Max power     35.46A
Open-Circuit Voltage     24.22V
Short-Circuit Current    41.90A

To have a longer battery life, I want to charge the battery at maximum 20A (@10 hours) in the first bulk charging constant-current (CC) stage. There's no setting in the charge controller to set the maximum charging current. During the peak sun hours, I get up to 35A current. If a load of 200W or more (@12V) is connected, the battery charging current remains within limit. But if there's no load, battery may get charged at higher rate.

So I was thinking about making a variable current limiter device. A current sensor (e.g. an ACS758) will sense the current at the battery terminal and feed it to a microcontroller (e.g. an Arduino Nano) which should then limit the current by a calculated amount if the battery is receiving more than 20A. Is it possible using a MOSFET, or a digital potentiometer, or by creating a dummy load?

Note that the required power dissipation may go up to 21V x 15A = 315W (if done on PV side of the charge controller) or 12V x 15A = 180W (if done on battery side). But it depends on the time of day and the connected load; so requires a variable current limiter.

 

[kommando]

Buy another battery, connect in parallel to keep voltage the same and the amps 1/2 per battery so a 40A total charge is 20A per battery. You get double the storage and no faffing about getting some bespoke electronics to work.

 

[Irfan Latif]

Buy another battery, connect in parallel to keep voltage the same and the amps 1/2 per battery so a 40A total charge is 20A per battery. You get double the storage and no faffing about getting some bespoke electronics to work.

So if I increase solar power, should I keep adding batteries? E.g. to get 50A (@12V) at peak sun hours as I have a plan, I should have at least 500Ah battery to keep the charging rate below C/10. Battery is the most expensive and the fast-wearing component of the solar system (needs replacement every few years). 200Ah is already the double of what I need. So this approach isn't practical in my case. A better solution would be to get an expensive MPPT controller with more controls, which I don't want to go with.

Thanks anyway.

 

[kommando]

If you add more PV then to keep the charging rate down then you need more batteries. Lithium batteries are cheaper than Lead Acid and for the same AH give you more usable energy and take higher charge rates.

 

[OzSolar]

A better solution would be to get an expensive MPPT controller with more controls, which I don't want to go with.

But that will be much better solution in the long run. Victron has a large selection of very flexible MPPT charge controllers for under $200 USD. I do acknowledge that "expensive" a relative term.

 

[Vigo]

What is the current charge controller? Given the fairly small numbers involved here it would not be that expensive to just install a charge controller with a current setting built in.