Solar SLA battery charging

I have a 1.5 watt solar trickle charger and wonder if this can charge SLA batteries direct. The open circuit output is around 17 volts but SLA batteries normally need constant 13.8 volts. If I use a voltage stabiliser I am concerned that all my limited solar power will be lost. Any comments would be appreciated. TIA

You have to be really careful charging SLA batteries. When fully charged they are very sensitive to overcharging.

You are quite right in saying that the voltage must be limited (the current does too, but in your case that won't be an issue)

If you are planning to trickle charge then don't exceed 13.6v you can keep this voltage across the battery indefinately.

They will charge much faster at higher voltages (you can safely go to 14.5v) but (and this is a big BUT) you must reduce the voltage once fully charged or you'll seriously shorten the battery life - and good SLAs are expensive and over charging really does shorten their life.

I tend to use a home made 2 stage charging system on my SLAs ( my daughter has a fairly elaborate ride on electric car that i made that uses 4 12v 18Ah ones) it bulk charges at 14.4v until the current drops off ( a sign they are charged) then drops to 13.6 for the trickle charge indefinately. People differ in opion, but I go from bulk to trickle once the charging current has dropped to the battery's Ah rating/100 so in my case I go to trickle at 180mA (18amps/100) of course dropping the charging voltage from 14.4 to 13.6 reduces the current more too.

Although your solar panel is 17v on no load, you'll find that in the most part when attached over a battery this will drop significantly due to the relative difference in internal resistance of the SLA (low) and the solar cell (high) but when fully charged the solar cell will only take some 10mA charging current (often called a maintenance current) and even the smallest solar cells terminal voltage will then rise above safe levels (13.6volt in this case as it is fully charged)

It is quite tricky, you could use something passive like a few diodes to drop the voltage to a safe level of 13.6 volts, but this will never really drive much current into the cell, and in dim lighting conditions the voltage will of course drop further.

So called charge regulators that you can buy for this purpose are quite pricey and as you rightfully say will sap your already fairly small source of energy

To be honest you'd be better off with a leisure battery - wet cells like this are far far more receptive to mistreatment, this might be cheaper than a decent charger circuit. You can get massive 100Ah ones for not too much money.

Another options might be NiCad cells, these can take abuse and if you shop around can be had for next to no money ( I recently bought a load of 1.3Ah 24v drill packs from Focus for a fiver each!!)

How big is/are your SLAs anyway?

Hope this helps
-Dan

Thanks Toasty for your comprehensive info. I have enough knowledge to build a voltage reg using 7812 with a couple of diodes in the ground leg, but as you say this will sap the current. My SLA's are 7 aHr and I have several available. I would be interested in your circuit for 2 stage charger as I may invest in a larger solar panel if any go cheap on e-Bay (or the normal very high price drops)

I assume your charger is a Maplin L58BF or similar.
I connect one of these directly across a 7Ah SLA battery, have done so for several years, no problems. No other form of voltage regulation, series or shunt, is required.
If you were using a larger panel, then that would be different.