I want to use a solar cell to top up a 12 volt 2.3 Ah sealed gel cell and show state of charge with LED. (A bit more than a basic diode and dropper resistor).The solar cell will be connected all the time, so its output varies from 0 volts at night to 18 volts @ 180 mA in bright sunlight. Am I right in thinking that a gel cell has different charging needs to a lead acid battery? I think it needs about 13.8 - 14.0 volts as a maximum. I don't want to overcharge it with 18 volts all day. The battery is used to power a 12 volt fan, switched on by a thermistor circuit at 21 C.
Any ideas? Thanks

 

Like I told you on Tony's site.

Quite Simple, Place a Series diode between the solar cell and the battery, as well as a 15 Volt 5 watt Zener or even better, a 15 volt Amplifed Zener across the solar cell.

(If you need a schematic for an Amplified Zener, Just E-mail me a chemelec@hotmail.com)

The Series diode prevents voltage going back into the Solar Cell and the Zener acts as a Shunt Voltage Regulator, limiting voltage to 15 volts before the diode. This Results in 14.4 volts or close to it to the battery.

Variations of this can apply to many simular applications and voltages.

Gell Cells and Sealed Lead Acid batteries are no different than car batteries, Other than There Current Ratings. They should Normally be charged at 1/10 Capacity. I the case of your 2.3 A/H Battery, that means a Maximum charge rate of 230 mA. Since your solar cell only delivers 180 mA max, the current isn't a problem.

Take care........Gary

 

The battery could still get overcharged. Most manufacturers recommend removing the charger or reducing the output of the charger to between 13.5 and 13.8 when the charge voltage exceeds 13.8 and the charge current drops to .01C. In this case 2.3AH X .01 = 23Ma.

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