Why does my power supply act weird when being used to charge an auto battery? - Page 1
Forum section: electronics / beginners

author: jaunty
author_rank: Regular Contributor
timestamp: March 28, 2019, 03:17:46 pm
content: I have an HP 6237B triple output power supply ... i thought i might see if i could use it to charge my severely drained auto battery (12 volts etc etc - you know the type) - but when hooked up it seems i cannot get the output level beyond just over 11 volts (cant' adjust it lower either!) - i can only vary the current and current won't go up too high (1.5A max) - would this have to do with some sort of interaction between the battery and power supply? or am i getting erroneous readings off the supply meters in this application? anyone have any experience with this or can explain?I was thinking of using my cheap and cheerful chinese supply as a countercheck but didnt' want to blow it up

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 28, 2019, 03:34:46 pm
content: Your power supply is fine, it doesn't go below 11V because that is the voltage of the battery it is measuring, and it will not go above that because the internal resistance of the battery is so low that you have to put a lot more amps in there to raise the voltage and your power supply is not capable of doing that. On the other hand, if you leave it connected (and the voltage setting is above 11V) you will see that the voltage will go up when the battery gets charged.

author: jaunty
author_rank: Regular Contributor
timestamp: March 28, 2019, 03:44:27 pm
content: alright - yes hopefully it will go up LOL - though it got drained from sitting around for MONTHS (down to 7.5 V or so) so we will see if it's restorable ... it charged up to 11 ish volts after a few hours but seemed to plateau at that point and didn't seem to charge higher

author: james_s
author_rank: Super Contributor
timestamp: March 28, 2019, 03:49:35 pm
content: It may have a shorted cell, 11V is about where I'd expect a 5 cell lead acid battery to top out.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 28, 2019, 03:50:44 pm
content: Quote from: james_s on March 28, 2019, 03:49:35 pmIf it starts to boil at 11V then yes, that could be the case. Also check the fluid level.

author: Nusa
author_rank: Super Contributor
timestamp: March 28, 2019, 04:30:51 pm
content: 1.5 amps is basically a trickle charge for a car battery. Expect it to take 1-2 days to get a decent charge at that rate, assuming the battery is healthy enough to be charged.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 28, 2019, 06:20:19 pm
content: To be safe, when trying to use an adjustable power supply as a battery charger you should always have a forward biased diode in series with the battery. This will prevent any voltage from flowing back into the power supply. Monitor the actual battery voltage with a DMM across the battery terminals.One place where I worked they had 2 expensive supplies fail in a week when they used the supplies to float auto batteries 24/7. What was happening was there were spikes on the lines to the battery that were triggering the crowbar over voltage protective circuit in the supply. When triggered, there was basically zero ohms across a battery that could deliver over 100 amps and the battery always won, destroying the circuit board in the supply. Adding a single diode eliminated that problem.If the battery voltage isn't going above 11 volts I'd agree it looks like a battery problem. I would try charging the battery at a higher rate for a short period of time and see if the voltage jumps back up to around 14 volts. There is a possibility that one cell has a separator problem and one cell shorted out. Sealed lead acid batteries are notorious for dying if left 6 months uncharged and the separators will swell and you end up with cracked cases and a battery that will never charge again.

author: helius
author_rank: Super Contributor
timestamp: March 28, 2019, 06:47:28 pm
content: You need to pre-set the voltage setting on your power supply. The bulk charging voltage for a 12V flooded battery is 14.8 V at 25°C; at different temperatures this needs to be adjusted by −0.005 V/°C. After the voltage is set, connect to the battery while avoiding sparks over the battery (i.e. connect leads to the battery first and the power supply last). At the bulk charge voltage the battery should not gas noticeably. If you hear gassing, there is a problem, perhaps a shorted cell as others have suggested. After bulk charging, the cells should be watered to the appropriate level (not overwatered) and equalized, although modern lead-calcium batteries do not provide an easy means to add water and so you can't equalize.If your power supply does have a crowbar circuit, definitely follow Arthur Dent's advice about the diode. You will need to compensate for the 0.7V diode drop in the initial power supply setting. From my research, the 6237B does not have a crowbar, however.