how to block charge from ups to battery - Page 1
Forum section: electronics / beginners

author: fxkl47BF
author_rank: Contributor
timestamp: March 08, 2023, 06:46:04 pm
content: 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?

author: Terry Bites
author_rank: Super Contributor
timestamp: March 08, 2023, 11:43:50 pm
content: TE make just the job: Plug-in Mini ISO Relays >Power Relay F4

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:49:50 pm
content: 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.

author: shapirus
author_rank: Super Contributor
timestamp: March 09, 2023, 08:45:33 pm
content: 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?

author: fxkl47BF
author_rank: Contributor
timestamp: March 10, 2023, 12:35:50 am
content: Quote from: shapirus on March 09, 2023, 08:45:33 pmCan 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.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 01:10:13 am
content: This would probably do the trick.https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-100BGQ100

author: shapirus
author_rank: Super Contributor
timestamp: March 10, 2023, 11:23:37 am
content: Quote from: fxkl47BF on March 10, 2023, 12:35:50 amThis 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?

author: fxkl47BF
author_rank: Contributor
timestamp: March 10, 2023, 12:48:20 pm
content: Quote from: james_s on March 10, 2023, 01:10:13 amThey're pretty cheap.I'll order a couple.Do I need one in both lines?

author: fxkl47BF
author_rank: Contributor
timestamp: March 10, 2023, 12:51:04 pm
content: Quote from: shapirus on March 10, 2023, 11:23:37 amI haven't checked that.It's at a remote site.I'll check that when I get back out there.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 09:19:58 pm
content: Quote from: fxkl47BF on March 10, 2023, 12:48:20 pmI 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.

author: fxkl47BF
author_rank: Contributor
timestamp: March 11, 2023, 02:50:21 pm
content: i'm sure i'm not fully understandingthe diode will have a 1 volt drop between the batteries and upsthe ups shuts down when the batteries get to about 22 voltsso with the diode the ups will shut down when the batteries get to about 23 voltsin order to make thing more convoluted, a bit rube goldbergi thought of adding two smallish batteries connected straight to the upsand two large batteries connected to the ups through a relayif the line power fails the small batteries will hold long enough for the relay to pull in the large batteriesthen when the line power returns the relay will drop out and the external charger can charge the large batteriesand the ups will recharge the small batteries

author: geggi1
author_rank: Frequent Contributor
timestamp: March 11, 2023, 03:17:52 pm
content: Use schottky diodes because they will have a lower voltage drop.

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2023, 08:12:37 pm
content: 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.

author: fxkl47BF
author_rank: Contributor
timestamp: March 11, 2023, 08:33:51 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2023, 09:15:54 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: March 11, 2023, 09:47:21 pm
content: Quote from: fxkl47BF on March 11, 2023, 02:50:21 pmThat'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.