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My solar power system contains a lead-acid battery but as soon as I use the inverter to power some load, the voltage drops instantly by 1 volt.

Why does this happen? And is it proportional to the load (bigger load = bigger voltage drop)?

JYelton
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bob dylan
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    Did you try to search for this information on your own? For example reading https://electronics.stackexchange.com/questions/9183/why-do-battery-power-supply-supply-voltages-drop-under-a-load – pipe Jul 28 '19 at 11:11
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    Thank you but I already checked both before posting : no and no. Check "Andy aka" post for an amazing answer. – bob dylan Jul 28 '19 at 11:33
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    A word about closing this question: so far it has received 4 votes to close based on it being a duplicate of a question about LiPo batteries (and not lead acid batteries) so I don't think on that basis it should be closed. Also, the so-called duplicate question is already closed so, it might be a disservice to close this question and misses the point about what this site is all about. The question is also about the initial rate of discharge voltage being high and this, is not adequately covered in the linked question(s). – Andy aka Jul 28 '19 at 11:44
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    Just to weigh in on that, it's not the first time I've seen that in stack "Electrical". Often people are voting to close / put on hold without reading or even knowing anything about the subject of the question. (Just copy pasting the question in their search bar ?) Maybe it's a recurrent problem that should be addressed. – bob dylan Jul 28 '19 at 11:55
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    I’ve nominated for a re open – Andy aka Jul 28 '19 at 14:59
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    So my question is a duplicate of a closed question about Lipo batteries with no accepted answer. Nice ! – bob dylan Jul 29 '19 at 11:57
  • Currently the close vote queue is at 170 or so questions and it may be understandable that with this backlog some reviews are not as thorough as they can be. In any case it is reopened now and you have an accepted answer. – crasic Aug 02 '19 at 08:26

1 Answers1

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Take a look at this graph from here: -

enter image description here

From All About Batteries, Part 3: Lead-Acid Batteries.

It's a typical 12 volt lead-acid battery discharge characteristic and it shows the initial drop from about 13 volts to around 12 volts occuring in the first minute of a load being applied. Thereafter, the discharge rate doesn't unduly affect the output voltage level until the battery gets quite depleted of stored energy.

This site explains in detail why that initial drop of terminal voltage is steep compared to the much slower drop in voltage that occurs afterwards: -

When a current is being drawn from the battery, the sudden drop is due to the internal resistance of the cell, the formation of more sulphate, and the abstracting of the acid from the electrolyte which fills the pores of the plate. The density of this acid is high just before the discharge is begun. It is diluted rapidly at first, but a balanced condition is reached between the density of the acid in the plates and in the main body of the electrolyte, the acid supply in the plates being maintained at a lowered density by fresh acid flowing into them from the main body of electrolyte. After the initial drop, the voltage decreases more slowly, the rate of decrease depending on the amount of current drawn from the battery.

Andy aka
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  • Amazing. Thanks ! – bob dylan Jul 28 '19 at 11:34
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    An additional question... Do you know if this voltage is proportional to the Ah of the battery or unrelated ? For example if I put several batteries in parallel will it change the voltage drop ? – bob dylan Jul 29 '19 at 11:55
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    @bobdylan Sorry but I don't know that answer. You might have to go on battery supplier's websites and make comparisons. – Andy aka Jul 29 '19 at 12:22
  • It is, according to the answer of https://electronics.stackexchange.com/questions/57508/how-does-one-determine-lead-acid-battery-end-of-life . `If you can't do a full capacity test, simply measure the instantaneous voltage drop with a known large load, and again graph the results.` – bob dylan Jul 29 '19 at 14:08