Battery Charger for Solar Panel

Thread Starter

tofino

Joined Feb 18, 2011
13
I have a 90 watt solar panel and 100A lead acid battery feeding a 12V DC led light bulb from dusk to dawn. There is a controller with low voltage disconnect at 11.5V and low voltage reconnect at 12V, presumably this function is to prevent the battery from being over drained.

This tiny system works OK in the summer months, but not so in the winter months, especially when there are days of snow. There is not enough sunlight to charge up the battery. Very often, the battery just goes dead.

One solution I can think of is to tie the battery to a battery charger at all times. Nowadays battery chargers can cut itself off when the battery is full.

My question is whether there is a way to activate the battery charger only when the battery goes down to say 11.5V or higher and deactivate when the battery is full, thereby making full use of the solar energy stored, Thanks.
 

Dodgydave

Joined Jun 22, 2012
8,680
You can leave the charger connected all the time, sla batteries can be trickled charged, just put a diode in series with the charger to drop the voltage to 11.5V, then when the sun shines your solar panel takes over.
 

Thread Starter

tofino

Joined Feb 18, 2011
13
You can leave the charger connected all the time, sla batteries can be trickled charged, just put a diode in series with the charger to drop the voltage to 11.5V, then when the sun shines your solar panel takes over.
Thanks. Intelligent battery chargers automatically go on standby when the battery is fully charged. With the diode in series, dropping the voltage to 11.5V, would this make the battery charger think the battery still needs charging, stays on even when the solar panel has taken over and the battery fully charged.
 

Dodgydave

Joined Jun 22, 2012
8,680
No the battery charger will continue to charge the battery on trickle, when the solar kicks in it will take over and trickle charge as normal or boost up the battery, when the solar panel is in darkness or winter months the battery charger takes over, you can't over charge a lead acid, they self regulate.
 

wayneh

Joined Sep 9, 2010
16,162
You might want 2 or 3 diodes in series. The charger will try to hold ~13.7V on the battery when the battery is fully charged. It will cycle on and off at that voltage. If you want the cycling on/off to start when the battery is at 11.5V, that's a voltage difference of 2.2V. That's very nearly 3 diode drops.
 

wayneh

Joined Sep 9, 2010
16,162
You want a diode with a current rating at least as high as your panel's short-circuit current. That current should rarely be seen, but a dead battery or an accidental short could send it that high. It usually costs a only little to add additional capacity. You obviously need a reverse voltage of at least 15V or so but any diode you look at will have that.
 

Thread Starter

tofino

Joined Feb 18, 2011
13
Thanks for the advises. I'll look up data sheets to find the appropriate diode. I am now confident that my batteries would survive the coming winter months.
 
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