I have an DC-AC Inverter/Charger system connected to AC utility power and a large bank of lead-acid batteries. This system includes internal relays that function as a transfer switch controlled by the inverter's own electronics. Control board's electronics seem to be internally connected after the relays so that they can use the DC battery power directly when the AC goes out and the unit switches itself to inverter mode. The problem with this design, however, is that means the batteries experience discharge even when AC utility power is present, because when the batteries are full, the charger is off. Unnecessary discharge cycles are caused by the inverter/charger itself as a result.

So I generally work around this problem by just manually switching off the DC breaker to the batteries when AC is present (which is most of the time) and let the electronics power themselves from the (disconnected) charger output. However, that solution still has problems, because sometimes when a momentary AC power surge happens, the relays switch over to the (disconnected) battery, and the unit gets stuck in that mode with no power for the electronics. When the AC comes back on, it can't switch itself back to AC, without first getting some power from the (disconnected) batteries.

What I would like to do to solve this problem is connect a small auxiliary SLA battery in parallel with the large lead-acid batteries, or switched alternatively, meant to provide only a small amount of power at all times for the unit's electronics. For that to work, I think I must use a current limiter, because the SLA battery is otherwise going to be way overloaded by the inverter? I wish the unit just provided a way to separately supply a backup DC power source for the electronics, but that doesn't seem to be the case (unless I start studying the control board itself). Would a relay on the SLA battery to switch it on/off opposite of the main battery bank disconnect switch also be a good idea?

 

That "small auxiliary power source" can be what is commonly called a float charger. The charger voltage is adjusted to what is known as the recommended float voltage, a voltage level that the manufacturer tells you will not damage the battery when constantly applied. In the TS case it would be providing that "small amount of power" to keep the monitor electronics active. The float supply will need to be powered from the same mains connection as the main battery charger so that a power failure will remove the float voltage as well. So by limiting the voltage you will also limit the current. So the float supply will probably deliver a bit over 12 volts at much less than one amp. A cheap and reliable solution.

 

That is an interesting solution that might work.

It's not quite that simple though, since we're dealing with a 24V system, though I'm sure the electronics probably step it down to something like 3V and probably will work just fine with 12V input too. Connecting a 12V float charger in parallel with a 24V charger does concern me though. Using a relay might be an answer to that, as the float charger only needs to be connected to the inverter/charger when the DC is totally absent.

Regarding float charging, the inverter/charger does have 24V float charge mode, but the problem is leaving the batteries connected and on continuous float charge all the time at 26.8V (13.4V x 2), for years, seems to overcharge the batteries and reduce their life too.

 

I have an DC-AC Inverter/Charger system connected to AC utility power and a large bank of lead-acid batteries. This system includes internal relays that function as a transfer switch controlled by the inverter's own electronics. Control board's electronics seem to be internally connected after the relays so that they can use the DC battery power directly when the AC goes out and the unit switches itself to inverter mode. The problem with this design, however, is that means the batteries experience discharge even when AC utility power is present, because when the batteries are full, the charger is off. Unnecessary discharge cycles are caused by the inverter/charger itself as a result.

So I generally work around this problem by just manually switching off the DC breaker to the batteries when AC is present (which is most of the time) and let the electronics power themselves from the (disconnected) charger output. However, that solution still has problems, because sometimes when a momentary AC power surge happens, the relays switch over to the (disconnected) battery, and the unit gets stuck in that mode with no power for the electronics. When the AC comes back on, it can't switch itself back to AC, without first getting some power from the (disconnected) batteries.

What I would like to do to solve this problem is connect a small auxiliary SLA battery in parallel with the large lead-acid batteries, or switched alternatively, meant to provide only a small amount of power at all times for the unit's electronics. For that to work, I think I must use a current limiter, because the SLA battery is otherwise going to be way overloaded by the inverter? I wish the unit just provided a way to separately supply a backup DC power source for the electronics, but that doesn't seem to be the case (unless I start studying the control board itself). Would a relay on the SLA battery to switch it on/off opposite of the main battery bank disconnect switch also be a good idea?

The SLA battery should be connected through a current limiter to prevent it from being overloaded by the inverter's demands if the primary battery bank is disconnected. A suitable current limiter can be a diode to prevent backflow from the SLA to the main battery bank when they are connected in parallel. Or you can use a resistor that limits the current to a safe level for the SLA battery.

 

That is an interesting solution that might work.

It's not quite that simple though, since we're dealing with a 24V system, though I'm sure the electronics probably step it down to something like 3V and probably will work just fine with 12V input too. Connecting a 12V float charger in parallel with a 24V charger does concern me though. Using a relay might be an answer to that, as the float charger only needs to be connected to the inverter/charger when the DC is totally absent.

Regarding float charging, the inverter/charger does have 24V float charge mode, but the problem is leaving the batteries connected and on continuous float charge all the time at 26.8V (13.4V x 2), for years, seems to overcharge the batteries and reduce their life too.

OK, please forgive my error! I had assumed that the battery supply voltage was 12 volts, not the two batteries in series providing 24 volts. So the correct float charge voltage would be near 24 volts, to support the series connection. So you will need a 24 volt adjustable supply, not the 12 volt supply. The principle is the same, to provide just a small charging current at the recommended voltage per battery.
Also, the float charge voltage should be adjusted to a lower value if that voltage seems to be excessive. No reason to stay with an incorrect setting. Correcting an error is almost always the better choice..