# Lead Acid Batteries: What voltage is considered "Charged"?



## KiwiEV (Jul 26, 2007)

This is a question I've had since I've installed my new batteries. Just what voltage should each 12V battery be at when it's charged?
Also, at what voltage would the battery be considered "Flat"? 

I'm assuming the "Charged" voltage of a standard 12V wet lead-acid is is 13.8 Volts. Is this correct? If so, what voltage would 50% DOD show?

They're probably very simple questions for a pro, but useful info for simple people like me. If anyone has the answers it might even be good in the Wiki too.


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## david85 (Nov 12, 2007)

I think the general rule with batteries, is that once the voltage drops below the rated capacity, they are considered discharged, and withmost battery chemistries (not just lead-acid) the voltage will tipically drop off rather quickly after you fall past the rated voltage.

In the case of lead acid, 13.8 is about what you can expect, although there will me some discrepancy from one cell to the next. If the battery cannot hold ~13v it is probably tired, although where I live, cold wether could do that as well.


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## houseoffubar (Nov 18, 2007)

Gav, As David says 13.8v is about right for a freshly charged battery. Most alternators in cars (the smelly ones) charge their battery at 14.4volts. After being shut off the battery will quickly drop to about 13.8, and may go down over a few hours to about 12.9. As for the discharge this chart should help. I read it on the internet, so it must be true!


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## KiwiEV (Jul 26, 2007)

That's very handy, I might keep it in my EV's glovebox!

So that means with a "144v" system, fully charged would be 165.6V and 50% DOD would be 144.7 V. 
So it's a good idea never to let your pack go below 144V then and it will maximise it's life. That's very handy to know. Thanks mate!


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## helixev (Nov 7, 2007)

According to the Deka sealed battery manual the battery charge voltage is dependent on temperature. Can anyone explain this?

these attachments are from the Deka sealed battery manual.


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## KiwiEV (Jul 26, 2007)

That's odd. Why would the voltage increase as the temperature goes down? 
I've now got more questions than when I started...


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## david85 (Nov 12, 2007)

It might have something to do with the reduced efficiency (and higher internal resistance) at lower temperatures. My guess is that at higher temps, the lower internal resistance means that a higher charge would be unneccicary, and prahaps damaging.


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## KiwiEV (Jul 26, 2007)

Ok, can someone help me complete this table:

Battery Charge Chart:

165v 100%
? 95%
? 90%
? 85%
? 80%
? 75%
? 70%
? 65%
? 60%
? 55%
144v 50%

That coloured chart halfway down the page says that anything over 12.6v is fully charged. But I've always heard 13.8V is fully charged. Now there's a big difference between 12.8 and 13.8 volts in a battery. 
If someone knowledgeable could complete this table, it would be very beneficial to other EV'ers using 144v systems. I'll put it on the Wiki too.

Come on battery pros, please fill in the blanks.


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## Mr. Sharkey (Jul 26, 2007)

I've been holding off telling you this for a while, but battery terminal voltage IS NOT a very reliable or useful indicator of battery state of charge (SOC).

Those charts you are reading are compiled when the battery has had a chance to sit for a while and recover it's natural terminal voltage for whatever SOC it is at. A battery under load will have depressed readings. Remove the load, and the reading goes up. But does it go all the way up to a reading that you can relate to SOC? Not for at least 24 hours at rest.

The reason is that electrolyte in contact with the plates is depleted, along with electrolyte in close proximity to the plates. Electrolyte in the farther reaches of the battery will have a higher specific gravity. As these various strata of electrolytes mix and circulate (as best they can without outside influence), the terminal voltage of the battery increases, eventually reaching an equilibrium that could be interpreted into a SOC rating _for that particular battery's individual chemistry_. AGM and gel batteries will take longer to reach steady-state than flooded cells.

As you also know, battery temperature plays a big role in terminal voltage, and that temperature is always in flux, due to daily variations as well as heat generated by charge/discharge cycles.

Throw in battery efficiency changing with battery age, and you get a grab bag of results.

Yes, terminal voltage can be used to guess semi-accurately at SOC, particularly if the measurement, and it's correlating data is adjusted for the load and battery variables, but there are many more accurate and reliable devices available for actually measuring SOC. If you are serious about battery health, you'll purchase one of these and use terminal voltage only as an on-the-fly guesstimate.

Somewhere in my EV files, I have a magazine article about a Fiat EV conversion that was fitted with circulating pumps on each battery cell (flooded batts) that was intended to destratify the electrolyte as the car was being driven. It was claimed that the range of the vehicle was increased by quite a significant distance using the system. Seems like a lot of work to get a voltmeter reading of SOC, though.


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## KiwiEV (Jul 26, 2007)

I know what you mean about the depressed voltage levels under load. I've generally left the car for an hour or three before I check the pack voltage and there's always a difference.
I've heard of other methods of measuring State of Charge but let's be honest, our voltmeter's are the most common measurement device we have in our cars and it's what most of us use for on-the-fly measurements.

If I use 155V= full and 144V= 50% DOD, then fill in the blanks, this is what I end up with:

100% 155v
95% 154v
90% 153v
85% 152v
80% 151v
75% 150v
70% 149v
65% 148v
60% 147v
55% 146v
50% 144v

Unless someone wants to correct it, I'll keep this in the glovebox. If it's correct, please use it yourselves too! Obviously it's for 144v systems.


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