Measuring Li-ion battery SOC during charging - Page 1
Forum section: electronics / projects

author: Alinik
author_rank: Contributor
timestamp: October 03, 2025, 02:42:54 pm
content: Hi everyone,I have a Li-ion battery pack with an integrated BMS. I want to monitor the state of charge of the battery by simply connecting a voltmeter to the positive and negative terminals.The problem is that when I connect the charger, the voltmeter reads the charger’s output voltage instead of the actual battery voltage.I thought about adding a diode in series with the charger’s positive lead, so that the voltmeter would only see the battery (except for the diode drop). But I’m not sure this is the best or most accurate method.My question is: what’s the proper way to read the true battery voltage even while it’s charging?Would a diode work (Example 2 figure), or should I use a follower op-amp buffer (Example 3 figure), or maybe the right approach is to read SOC directly from the BMS?Thanks in advance for your advice!

author: tunk
author_rank: Super Contributor
timestamp: October 03, 2025, 04:37:11 pm
content: No expert, but what do you mean by "actual battery voltage" and "true battery voltage"?

author: Peabody
author_rank: Super Contributor
timestamp: October 03, 2025, 09:06:02 pm
content: The charging voltage *IS* the actual battery voltage when it's being charged. If you want to know what the battery voltage would be without the charger, then you'll have to disconnect the charger from the battery. I don't think the diode does anything for you, and would probably mess up the charging.You might get some useful information if you can determine that the charger has switched into the constant voltage state, and then perhaps your BMS has some way to tell you what the charging current is at the moment. The more the charging current has dropped from the maximum, the more charged the battery is.

author: inse
author_rank: Super Contributor
timestamp: October 04, 2025, 06:25:02 am
content: If SOC could be determined by voltage measurement only, there wouldn’t be put so much effort in methods like coulomb counting.If you look at the discharge curve of a Li-Ion battery, there is a range where there is little dependence between charge state and cell voltage.What opportunities does the BMS offer? Often they only supply protection for over/undervoltage, temperature, overcurrent and cell balancing.

author: Alinik
author_rank: Contributor
timestamp: October 06, 2025, 07:16:44 am
content: Thank you for your replies.Is there anything commercially available that I can connect in series or parallel between the charger and the batteries to measure the state of charge?

author: inse
author_rank: Super Contributor
timestamp: October 06, 2025, 09:27:19 am
content: Sure, simply do a search on “battery coulo meter“

author: radiolistener
author_rank: Super Contributor
timestamp: October 06, 2025, 09:44:14 am
content: When you connect a voltmeter directly across the battery terminals, you’re already measuring the actual battery voltage. The key is to connect the voltmeter as close as possible to the battery terminals themselves — not somewhere along the wires leading to the charger or the load — since voltage drops along those wires can mislead your readings. It’s best to run a dedicated pair of sense wires directly to the battery terminals.What you seem to want to observe is the open-circuit (EMF) voltage of the battery — that is, the voltage without any charging or load current. However, in Li-ion systems, the battery voltage is intentionally influenced by the charger during charging. The charging process defines the state of charge (SOC): the voltage naturally rises under charge current and the charger must stop when the cell voltage reaches 4.2 V (or the pack equivalent). It should never exceed this limit — even by 0.05 V — as this indicates improper charger regulation.In short, a diode won’t help, it will only introduce voltage drop and reduce charging efficiency. If you need to know the SOC accurately, the correct way is to read it from the BMS, which measures both voltage and current to estimate charge state.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2025, 11:05:23 am
content: Measuring SoC is complicated. Off-the-shelf solutions are expensive specialized instruments and even then need understanding to set up correctly. Battery "fuel gauge" ICs exists, and of course breakout/demo boards using them, but still configuring them requires care. There is no simple and quick solution, you need to dive into the rabbit hole.100% and 0% are somewhat easier to measure. That's why typical "fuel gauge" design is to reset an integrator at 100%, then integrate charge, and calculate SoC% by dividing integrated value (value now) by maximum capacity (which has to be measured with said integrator). This is prone to serious drift if battery is not occasionally fully charged or discharged for integrator reset.And even fancy systems are well known to sometimes get the SoC colossally wrong. It's not an easy engineering task at all.