Battery Charging education Requested - Page 1
Forum section: electronics / general

author: Purduephotog
author_rank: Regular Contributor
timestamp: August 13, 2025, 03:20:26 pm
content: I am having some issues understanding what is happening in my system. Previously I had a dedicated charger- it went poof.I have 3x LiFePO4 (8x cells) batteries in parallel. They're limited to 6A each. Each pack has an integrated BMS (which does function by shutting down the pack)I currently have them hooked up to a lab bench PSU, set to 29.2V and 3.5A max, in Constant Current mode.The supply is indicating it is feeding in 1.4A, and a clamp meter is close enough for this to be legit.The voltmeter I have across the battery is reading 26.1V, however. The PSU reads 29.2.This is a new benchtop supply on loan and I may be using it wrong.Would someone far smarter and intelligent than me please tell me what I'm doing wrong, because none of these numbers make sense.Most grateful today.Edit: PSU is the Gophert 1602The display indicates it is in CV mode.

author: thephil
author_rank: Regular Contributor
timestamp: August 13, 2025, 05:49:38 pm
content: Sound like you are observing some voltage drop across the leads. Using bigger leads or a PSU with sense leads should mitigate/fix that.

author: Purduephotog
author_rank: Regular Contributor
timestamp: August 13, 2025, 06:31:18 pm
content: Unfortunately not voltage drop- turning off the psu and the meter doesn't fluctuate.No, it seems the PSU is only putting out a much lower voltage instead of the 29.2 that I've set it to.

author: tooki
author_rank: Super Contributor
timestamp: August 13, 2025, 06:34:21 pm
content: Quote from: Purduephotog on August 13, 2025, 06:31:18 pmUse a multimeter to measure the output voltage directly at the power supply, with the load connected. What do you measure? And then again with the load disconnected?

author: Purduephotog
author_rank: Regular Contributor
timestamp: August 13, 2025, 08:21:18 pm
content: Quote from: tooki on August 13, 2025, 06:34:21 pmWhen the unit is powered on and the voltmeter connected across the battery leads it measures 26.1 (or whatever it's at now).

author: thephil
author_rank: Regular Contributor
timestamp: August 14, 2025, 07:57:41 pm
content: What is the reading at the terminals of the power supply with and w/o the battery connected?

author: Purduephotog
author_rank: Regular Contributor
timestamp: August 15, 2025, 12:02:52 am
content: Quote from: thephil on August 14, 2025, 07:57:41 pmWith the battery disconnected (the load) the DMM read 29.0V; The gophert PSU was set to 29.2V. When the battery was connected in the battery was at 27.2V.I do need to recalibrate/chase down the discrepancies at some point again.Thank you for your patience.

author: Psi
author_rank: Super Contributor
timestamp: August 15, 2025, 12:13:37 am
content: Quote from: Purduephotog on August 13, 2025, 06:31:18 pmCan you explain again why you think it's not voltage drop in the cables. It's not totally clear why you discounted this as an option.Note that it's possible to have a Vdrop is inside the PSU between the voltage read/set circuitry and the output terminals.The psu should have a Vsense wire connected to the terminal post to measure voltage at the terminal, but not all PSUs do.If a bad connection develops on the binding post or the wire is just too thing you might get this effect.

author: Purduephotog
author_rank: Regular Contributor
timestamp: August 15, 2025, 02:11:55 am
content: Excellent point- I discounted it because it would have been dropping 3 volts in 1 foot- and they've always worked before (that's a dangerous assumption).I will go order a new cable, this one premade with eyelets and the proper plug instead of the hodgepodge I built- and ensure it's 12 awg instead of the 14 I've got.It'll take a few days but I'll test it out.And thank you for hitting me in the head several times to make sure it's not the cable. I used to train and diagnose, and it was always the cable when I dealt with people.

author: Psi
author_rank: Super Contributor
timestamp: August 15, 2025, 11:24:10 am
content: You can always put a DMM set to volts across the cable from one end to the other while current is flowing and confirm it's near zero volts.If you are losing a volt or so when current is flowing it will be dead easy to see with a meter across a cable.