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Forum Index : Electronics : Lead Acid Charging... Help Me Understand
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InPhase Senior Member  Joined: 15/12/2020 Location: United StatesPosts: 178 |
I have a lead Acid forklift battery, about 1100 Ah. It was pretty cloudy this past week and the voltage was getting a bit low so I ran the generator to top it off. The voltage rose relatively quickly from 49 volts to 57.8 but it wouldn't go above that value. I was putting about 170 amps into it. After about an hour of generator time and it not rising any further, I shut the genny down. But then I noticed that the SCC was putting about 50 amps into the battery and the voltage was at 58.4. I don't understand why the battery wouldn't go above 57.8 with 170 amps but easily went over 58 V with 50 amps. Can someone help me understand what's going on? |
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wiseguy Guru  Joined: 21/06/2018 Location: AustraliaPosts: 995 |
If I assume the SCC was how you were measuring the battery voltage for both the generator and SCC voltages, maybe the following is occurring. When you were charging with the generator it was via a direct connection to the battery and no current is flowing in the SCC wiring (ok miniscule current for meter). When the SCC is now charging there is 50A flowing in the SCC wiring causing minor voltage drops so yes the SCC reports a slightly higher voltage at its terminals (58.4). But if you use a voltmeter directly at the battery terminals and then compare it to the SCC output terminals you should see a higher voltage (~0.6V) at the SCC when it is charging at ~ 50A. I believe the battery voltage was the same for both, those huge batteries have an inertia like the rock of Gibraltar it wont budge much and certainly not quickly, unless well undercharged or at the gassing point. Edited 2023-11-25 01:22 by wiseguy If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 152 |
I don't really have an answer but do have questions:- The 1st question is are you reading 57.8v and 58.4v using the same voltmeter in the same place (ideally on the battery posts)? 2nd q is: what time elapsed between reading the 57.8 @ 170A and 58.4 @ 50A? The implied elapsed time was 'not very much'. The reason for the 1st question is the voltmeters on different bits of kit may have calibration differences, and even if you're using the same voltmeter for both readings, with those currents flowing it's important to measure in the right place. The reason for the 2nd question is that if there's a significant time elapsed, then the battery could have cooled after its 170A charge or it could have equalised further on 50a over the elapsed time - but if it was a short time interval, then your voltage readings are not making sense - which is why you posted in the 1st place. Edited 2023-11-25 01:29 by mab1 |
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| InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178 |
Voltage drop is a factor but only maybe a tenth of a volt because of the size of the cables involved. The SCC has an independent voltage sense lead to the battery terminals and it agrees pretty close with my voltmeter. The SCC display actually showed 57.74 volts, the 57.8 was from the inverter display (which was doing the charging). Since the inverter is the source of the charge, it's display is probably a little higher. But regardless of the actual terminal voltage, wouldn't the voltage still rise instead of just hovering, even if it is being reported inaccurately by the display? As far as timing between readings, it was minutes. I shut the generator down and the battery voltage dropped to 55 volts as the SCC took up the slack. Then the sun came out a little and the voltage ticked up to 58.4 pretty quick. Edited 2023-11-25 02:12 by InPhase |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 152 |
When the scc was putting out 50a was it's voltmeter still in agreement with the inverter? I can't think of any reason why the scc would display the voltage on main cables rather than the sense wires, but i try not to assume anything. If the battery was fully equalised then the voltage would be fairly flat tbh; might even go down a tad if the battery was warming up. And it would be gassing vigorously. You're certain that the charge amps really were in the 170a ballpark on the generator? Not less than 50a? |
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| InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178 |
Yes, the charge amps were verified by my clamp ammeter as well as the fact that the input amps at 240 volts was around 45-50 amps. |
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| mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 152 |
Sorry if i seem to be asking a lot of stupid questions- it's not that i don't think you've checked the 'obvious', but i do fault-finding for a living and the number of times i ask over the phone "have you checked the circuit breakers? Are ALL the isolators turned on? Have you checked ALL the interlocks and emergency stop buttons?" Only to drive out there and get "oh, the breaker looked to be on when i looked at it" or "oh i didn't know there was a switch by the machine - the repair man must have turned it off" or "i didn't know there was a stop button round the back - i never go round there" So my 1st thing when i start looking is to check the obvious even though I've been told they've been checked - it's saved me a lot of time over the years, but now i can't look deeper until I know i've checked the basics. |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 506 |
Meter A strong suggestion, lead acid is very difficult to check state of charge via voltage alone.. sounds to me the batteries were quite flat and were just taking some good charge.. the meter above columb counts, so tells you whats coming in ( or going out) and adjusts total ah available to suit at a glance. pretty accurate and very handy to check the state of charge of your bank.. hope this helps.. I think it works !! |
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| zeitfest Guru Joined: 31/07/2019 Location: AustraliaPosts: 381 |
Not sure this is causing it, but there is a phenomenon where initial/very slow gassing from electrodes has a slight voltage hill ("overpotential"), which drops when the gas evolution becomes more pronounced and bubbles a lot, I believe it is due to the electrode surfaces inhibiting the actual physical gas formation. |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 411 |
Is that 170A a nice DC current? It might have a big ripple that can cause some bad readings? I would do the same test but reading the voltage at the battery terminals. Edited 2023-11-25 18:22 by nickskethisniks |
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| InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178 |
Thanks for the info fellas. I'll look deeper into it and report back. |
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| InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178 |
Thanks for the info fellas. I'll look deeper into it and report back. |
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| johnmc Senior Member Joined: 21/01/2011 Location: AustraliaPosts: 282 |
I do not think instant voltage for large traction batteries is very accurate but you should wait long enough (1/2 hour) for the battery to settle. Cheers john johnmc |
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