Is it possible to un-confuse a confused smart charger?

[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]

Maybe this is a case of the wrong tool for the job? A simple dumb charger would seem to be more appropriate.

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!

[Ian.M]

Smart chargers intended for fixed installations, designed for the marine and RV markets usually support end of charging float mode with significant load current, and are specified to do so, as supplying 12V 'house' power when mains shore or site power is available is pretty much a standard requirement for those markets.   

[madires]

How much does that approach shorten the battery's life span?

[richard.cs]

How much does that approach shorten the battery's life span?

The dumb approach? There is a tradeoff to it, where a float voltage too high eventually cooks them (especially SLA) and one too low gives you less capacity than you might have hoped. Lead acid is generally pretty forgiving though and I have a few setups like this with batteries 5-10 years old. For standby or starting use smart chargers are not especially common, most alarm panels, a lot of cheaper UPS's*, cars before around 2010 are mostly constant-voltage chargers.

*These do exhibit poor battery life, around 2 years. This is because they set the voltage pretty high and the environment tends to be hot. Make your own judgement as to whether high float voltages in UPS's are to squeeze a few % more runtime or sell more batteries.

[e100]

Smart chargers intended for fixed installations, designed for the marine and RV markets usually support end of charging float mode with significant load current, and are specified to do so, as supplying 12V 'house' power when mains shore or site power is available is pretty much a standard requirement for those markets.

Yes I see what you mean , for example https://www.ozcharge.com.au/collections/caravan-battery-chargers/products/12-volt-10-amp-battery-charger-maintainer-and-power-supply is basically a lab CV/CC power supply with other modes thrown in.

Seems like "battery maintainer" is the phrase to search for.

[floobydust]

This is a common problem with "smart chargers", that they get stuck in float charging mode as the final stage.
So any loads appearing on the battery are unnoticed and a battery can eventually go dead or get harmed.
Another RV/Caravan charger does a similar thing, stuck in pulsed float charging it never goes into bulk charge or equalization mode and with only float voltage the battery fails after 1-2 years. even though it can catch up overnight.

For an algorithm, you could shut off charging and look at battery voltage.
If the voltage or SoC is dropping, the battery has a load or a shorted cell.
Some smart chargers do this, they re-assess the battery in each stage to see if they need to roll back.
Example "Charge cycle restarts if voltage drops" example C-tek 8-stage charger, at the very last bottom note but who knows what voltage it has to drop down to?
The C-tek use cheap crappy capacitors so they aren't great quality and pretty overpriced, IMHO.
Optimate and Deltran do not seem to restart charging when in float mode.

[PlainName]

This is a common problem with "smart chargers", that they get stuck in float charging mode as the final stage.
So any loads appearing on the battery are unnoticed and a battery can eventually go dead or get harmed.

I can confirm this - I've twice had motorcycle batteries on long-term storage be completely trashed using (different) Optimate chargers. OTOH, a CTEK doing the same job has been no problem at all and the battery is still good several years on.