24V UPS using 18650 batteries that charges/discharges using the same bus - Page 1
Forum section: electronics / projects

author: dfindlay
author_rank: Contributor
timestamp: February 07, 2020, 04:06:09 am
content: Hi,I have some equipment that is powered by a 24VDC power supply typically drawing 2-4 amps. The load is directly connected to a power supply and I would like to design a UPS that is in parallel with the power supply/load so that the batteries charge from the power supply and if the power supply is cut, the batteries would start discharging. I would like to use 18650 batteries and my understanding is that I will need a charging circuit and a BMS. My challenge is that any charging circuit I can find uses a separate charging supply than the load the main PS and load are on. Is it possible to do this?If the charging circuit is powered by the same bus, when the UPS kicks in (main PS is lost) the battery would be supplying power, but it would also be trying to charge itself. Has anyone come across this situation before?Thanks in advance for your support!

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 07, 2020, 04:48:28 am
content: With a 6S pack including a BMS, all you need are two diodes and a resistor. The diodes are used in the usual "diode or" configuration, then a resistor is added from the output back to the battery for charging. The pack would end up at about 4V/cell, which is OK to leave connected indefinitely. If the PSU is OK with backfeed, the diode in series with it is not necessary. If you use a PSU that also allows limiting the output current, you could indeed simplify it to a direct parallel connection as you show.

author: dfindlay
author_rank: Contributor
timestamp: February 07, 2020, 09:55:21 pm
content: Hey NiHaoMike, thank you very much for your help here. As I was looking into your solution, I came across a BMS from Amazon that appears to balance charge and discharge on the same wires. I sketched this up based on this part (lh-hp07sa). Do you think this will work, and more importantly work safely? See attached updated sketch.https://www.amazon.ca/dp/B07R58FX1V/ref=cm_sw_em_r_mt_dp_U_.wBpEbJYMSP44Thanks in advance for your help, I really appreciate it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 08, 2020, 03:09:38 am
content: For that to work, the PSU must tolerate backfeed and limit the output current to within the charge rating of the battery. Otherwise, you'll need the diodes and resistor. That's also a 7S BMS when you want a 6S.

author: dfindlay
author_rank: Contributor
timestamp: February 08, 2020, 04:07:35 am
content: Ok Fantastic. Yes, the power supply can tolerate backfeed, but I might as well add a protection diode(D1). The batteries don't need to charge quickly, in fact, I would prefer them to charge slowly to avoid pulling too much current from the load. Does this diagram make sense?Also, I think that BMS can be used in any configuration from 3S-7S. Correct me if I'm wrong, but to use 6S I would just short out the 7S lead to the 6S right?Thanks!!

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 08, 2020, 04:15:21 am
content: D3 is unnecessary. 47R is quite a bit higher than would likely be sensible, 5-10R would be more like it. It would get quite warm so go for a 10W or 25W to have plenty of margin. (Or just use a car light bulb, which has a PTC characteristic to control the charging current better than a fixed resistor.The BMS makes no mention of how to use it on a 6S pack so it only supports 7S.

author: dfindlay
author_rank: Contributor
timestamp: February 08, 2020, 04:43:44 am
content: Right, that makes sense about D3 and the lower resistor rating. I guess in interpreted the BMS wiring diagram incorrectly thinking that the omitted cells meant they could be removed for fewer overall cells. In the event of using 7S battery configuration, the 24V supply would just charge the bank to 24V (minus the diode Vf) right? This might be desirable since this unit will be charged 99% of the time.Due to the voltage drop across the diodes (D3 during charging and then add D2 during discharge), I think I would add a step-up DC-DC voltage regulator and this would help maintain the 24V even when the banks overall voltage drops.Would this be a better suited BMS/charger?https://www.amazon.ca/dp/B07QQT99P8/ref=cm_sw_em_r_mt_dp_U_oJJpEb07ESS00Thanks!

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 08, 2020, 04:51:04 am
content: That BMS will work great. You'll only need the boost converter if the load requires an exact 24V.

author: dfindlay
author_rank: Contributor
timestamp: February 08, 2020, 04:55:48 am
content: Ok great, I'll go with that. Thank you so much for your help!