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Is it possible to un-confuse a confused smart charger?
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e100:
I have a DIY uninterruptable power supply for my 12v network gear consisting of a 40Ah SLA battery continuously charged from a modern generic "5 stage" 4 amp lead acid smart charger.

The battery is permanently connected to the network gear without any change over relays or fancy stuff so the system is as simple as possible. The gear continuously draws about 2 amps in total which should be no problem for a charger rated at 4 amps.

The problem is that the smart charger prematurely goes into trickle charge mode when it should be pumping in the amps to counteract the 2 amps being continuously drawn out. Trickle charge mode for this particular charger means it puts out a 4 amp pulse for about a second, then nothing for about 1.5 seconds and so on.

The net result is that the 'amps in' is less than the 'amps out' so the average voltage keeps dropping. I was expecting it to stabilize at around 13.8v but instead it got down to 11.8v at which point I disconnected it and had to use my lab power supply as a constant voltage float charger to bring it back up to a sensible voltage.

I'm saw similar problems with another 5-stage another smart charger so I suspect that this is a general problem with 5-stage chargers that haven't been designed to charge a battery that is actively being used as a power source. In a normal charging scenario the battery would either be on the bench and not connected to anything or sitting in a car that is perhaps drawing a few milliamps to keep the locking system alive etc.

So is there a way to un-confuse this charger so that I can continue using it this way or am I basically out of luck.

PlainName:
Maybe this is a case of the wrong tool for the job? A simple dumb charger would seem to be more appropriate.
mc172:
You will probably find that the charger does an on-off cycle for all charging modes, not just trickle mode.
richard.cs:

--- Quote from: dunkemhigh on March 02, 2022, 11:32:37 am ---Maybe this is a case of the wrong tool for the job? A simple dumb charger would seem to be more appropriate.

--- End quote ---
Agreed, a dumb constant-voltage (possibly with temperature compensation) charger with a current limit is probably more appropriate here. It won't recharge as quickly after use as a charger with an absorption phase, and it won't do an automatic equalising charge (but SLAs don't like that anyway), but these don't seem likely to be a problem.
SilverSolder:

This depends on the algorithm programmed into the smart charger, so how it acts will depend on each brand/model of charger.

Some smart chargers behave well in this scenario.  For example, I have a "Vector" brand charger here that I connect to my car battery when working on the car and playing the car stereo...   The stereo draws a lot of current sometimes, 10A or more, while drawing little or no current at other times.   The Vector charger happily keeps the current flowing. 

The problem is that this behaviour is not part of the spec, so you almost have to buy one to see if it works!

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