how to block charge from ups to battery

[fxkl47BF]

I have a oldish ups that has a plug for external batteries. It's 24 volts and has a maximum draw of 60 amps. I would like to plug in some large external batteries and use an external charger. I believe I need a way to block the ups from trying to charge the batteries. I tried a test and the charge voltage coming from the ups seems to confuse my external charger. Would an automotive isolator work for this if I can find a 24 volt one?

[Terry Bites]

TE make just the job: Plug-in Mini ISO Relays >Power Relay F4

[LooseJunkHater]

I'm definitely no expert, but I wonder if a simple diode would work? Maybe you can give it a try and see if it works at idle, and if it does, get yourself an "ideal diode" which can handle the high currents of the UPS.

[shapirus]

Yeah, a diode (at a low load) is a good way to verify whether the UPS will stop confusing your external charger. If it will, then either a relay or an ideal diode is the way to go.

However, it's quite interesting why and how exactly the external charger gets confused. Can you elaborate on this?

[fxkl47BF]

Yeah, a diode (at a low load) is a good way to verify whether the UPS will stop confusing your external charger. If it will, then either a relay or an ideal diode is the way to go.

However, it's quite interesting why and how exactly the external charger gets confused. Can you elaborate on this?

Can you point me to a diode that might work?
The ups charger is very weak.
I hooked up a couple of small auto batteries and discharged them to about 11.5 volt.
It took the ups almost a week to recharge them.
But the high voltage coming from the ups seems to tell my external smart charger not to charge.

[james_s]

This would probably do the trick.

https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-100BGQ100

[shapirus]

It took the ups almost a week to recharge them.
But the high voltage coming from the ups seems to tell my external smart charger not to charge.

This doesn't add up.
The voltage that the external charger sees is equal to what is present on the battery terminals, which in turn equals to what the external charger has to create on its output terminals.
In other words, all voltages are equal, since both chargers and battery are connected in parallel.

What is the voltage on the the output terminals of the internal UPS charger when no battery is connected?

[fxkl47BF]

This would probably do the trick.

https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-100BGQ100

They're pretty cheap.
I'll order a couple.
Do I need one in both lines?

[fxkl47BF]

What is the voltage on the the output terminals of the internal UPS charger when no battery is connected?

I haven't checked that.
It's at a remote site.
I'll check that when I get back out there.

[james_s]

They're pretty cheap.
I'll order a couple.
Do I need one in both lines?

I wouldn't think so. Doing that will have the disadvantage of doubling the forward drop. I intentionally selected a diode with the lowest drop I could find that met the other requirements and was reasonably priced.

[fxkl47BF]

i'm sure i'm not fully understanding
the diode will have a 1 volt drop between the batteries and ups
the ups shuts down when the batteries get to about 22 volts
so with the diode the ups will shut down when the batteries get to about 23 volts

in order to make thing more convoluted, a bit rube goldberg
i thought of adding two smallish batteries connected straight to the ups
and two large batteries connected to the ups through a relay
if the line power fails the small batteries will hold long enough for the relay to pull in the large batteries
then when the line power returns the relay will drop out and the external charger can charge the large batteries
and the ups will recharge the small batteries

[geggi1]

Use schottky diodes because they will have a lower voltage drop.

[james_s]

The diode I suggested is a Schottky diode.

Voltage drop is wasted energy, some of the power in the batteries will be burned up as heat in the diode, and adding 1V of drop will cause the UPS to shut down when the battery is still 1V higher than it otherwise would. Doubling this to 2V will exacerbate the issue.

[fxkl47BF]

Someone else suggested using the diode to block the ups charge and add a relay to bypass the diode when the line power was off.
The diode you suggested would work I just need to find a suitable relay.
SSR's are much cheaper than mechanical relays but I've fried several with high inrush current.

[james_s]

SSRs have the same problem diodes do in that they are semiconductors and have a voltage drop. They are used mostly when rapid cycling is required.

[Gyro]

...

in order to make thing more convoluted, a bit rube goldberg
i thought of adding two smallish batteries connected straight to the ups
and two large batteries connected to the ups through a relay
if the line power fails the small batteries will hold long enough for the relay to pull in the large batteries
then when the line power returns the relay will drop out and the external charger can charge the large batteries
and the ups will recharge the small batteries

That's actually the best idea I've seen in this thread. You should include a(nother) fuse to protect against the possible fault situation, eg. charger fault, where one pair of batteries is fully charged and the others empty.