A quick question on backup power

I've been looking into this sort of thing a little bit, as I need to do something about the solar setup in my outbuilding (wife's art gallery) now that the charge controller seems to have died.

From what I understand, as long as the maximum charge amps of the solar cell is at a rate small enough compared to the total battery capacity (ie, the equivalent of a trickle charge), you don't actually need any circuitry (other than that diode I guess) - the battery will just dissipate the extra energy in the form of a small amount of warmth.

But I'm using basic lead-acid batteries in my setup, so that might not apply to your NiCd battery.

I could also be very very wrong, and if so I hope someone will correct me before I blow up my wife's art gallery

Cheers,

-Nico

Hi Nico,

I think you're right, I think some Lead Acid, e.g. possibly burglar alarm batteries, can be charged like this as long as you like. One thing possibly worth

doing would be to have a cut-off that constantly monitors the battery voltage, and cuts off the solar cell (or reduces the charge current)

if the voltage exceeds approx 13.8-14V (usually marked as the float voltage in the datasheet for the battery) for a 12V battery.

For NiCd, some batteries (usually the larger ones, e.g. 'D') can be indefinitely trickle-charged (i.e. topped up) as long as the current is kept low (e.g. a series resistor), and probably trickle-charging like this doesn't apply to newer rechargeable type chemistries.

If you wish to charge at a higher rate too (i.e. when the solar cell can supply the power, and when the battery does need more than trickle charging),

then for both lead-acid and NiCd,  you know when the battery is charged by looking at dV/dT which will rise as a symptom of end-of-charge being reached (easy to monitor this with a dedicated IC I guess). If there are cold conditions, NiCd was preferred over Lead Acid for more efficient charging in the cold, but maybe manufacturers have resolved this nowadays.

In the past some older charger ICs just relied on dV/dT, some on just time, etc., but a combination (including temperature) is good

for safety, for NiCd and NiMH batteries. Modern ICs support the combination of charge termination conditions (with no charge termination, if the current is too high, even large NiCd batteries will eventually pop (I guess as you say, the heat doesn't reach the steady-state at this high level of energy) and has to go somewhere : ) I saw that occur once, not pretty!

Li-Ion are super-sensitive and possibly best served with specialized charger ICs always.

Thanks for the advice, Shabaz

I've decided to just buy a controller for my setup - easiest way to make sure all is well, and it turns out their not that expensive anymore.

Thanks for the advice, Shabaz

I've decided to just buy a controller for my setup - easiest way to make sure all is well, and it turns out their not that expensive anymore.