High Battery Voltage

So going back to our last event this past weekend and from over the summer, I remembered that the battery voltages we were getting were way over 12v (I recall one match a while ago where it was 13.84v). Obviously every battery you pick up wont be exactly 12v but I have a sneaking suspicion something here isn’t right. The batteries seem to preform fine during a match, is it us? Are our batteries past their prime? Is this normal?

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13.x voltage is pretty normal for these batteries after a charge. 13.6v is what most of ours go to.

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Can you please share what type of battery you use? And maybe an insight as to how you wire/crimp your batteries?

Our best batteries are at ~12.8V right off the charger, and I suspect there’s something we could be doing better.

What charger are you using? We had some “smart” NoCo chargers that didn’t get them nearly as high as the simpler ones. Also, what are you using to measure the voltage?

A lot of lead acid battery chargers go into a maintainer/trickle charge mode once the battery is fully charged. It’s not uncommon for batteries to go above 14v when left for a while. It’s important to note that more voltage doesn’t necessarily mean more power/capacity. You’ll find it drops to normal voltages quite quickly as the actual amount of charge stored between 13 and 14 volts is quite small.

Yup. Dead batteries drop more voltage while under load, fresh ones can sustain their output under more stress.

Our batteries average around 13.6V right off the charger. We check all of our batteries somewhat regularly, especially before competition, with a battery beak, but any battery which does not reach at least 13V invites additional scrutiny.

Both a dead battery and a good battery will display the behavior I described. If you look at a voltage vs. state of charge graph for a SLA battery like this one you’ll see once the battery hits it’s fully charged state you can continue to feed it charge current and you’ll see the voltage increase rapidly.

While the voltage may continue to go up it’s not actually storing significantly more energy as it’s already at maximum capacity. You’ll find this voltage (usually between 13.6v and 14.8v depending on the battery/charger setup) drops off rapidly when discharged and will quickly level off at the proper voltage for a fully charged battery (~13.6v)

12 volt nominal voltage =/= 12 volts battery voltage

13+ should be the goal and expected for good batteries.

We’ve used the 3 bank charger Andymark sells for a few years now, we run 2 gauge wire for our batteries so we use a battery beak and basically some 6 gauge wire with a SB50 connector on one side and a SB120 on the other to connect the two.

What is your battery voltage 24 hours after coming off the charger?

Admittedly, probably not great, but we use these and these.

We measure our voltage using a battery beak, and also look at the driver station voltage.

Did you consider the possibility that the smart chargers were charging the batteries properly, and the simpler ones were not?

Probably, but we prioritize a few percent performance increase and charging speed from aggressive charging than lifetime. Seeing “130%” on the battery beak is a whole lot nicer than “97%” even if the actual difference isn’t much.

If anyone reading this has actual test data quantifying such an increase in performance, please do share it.

Meanwhile, to those in this thread who mentioned charge levels of 13.4 volts and higher fresh off the charger: what is the voltage 24 hours after coming off the charger?

Depends on the battery, why does it matter?

The “resting voltage” is a better indication of state of charge, when all of the battery has reached room temperature and the tiny gas pockets created along the plates during charging have had a chance to dissipate into the electrolyte. Our 18 ampere-hour batteries tolerate being charged at 6 amperes but it’s not the nicest thing you could do to them.

See Battery Life thread my 05-05-2011, 09:29 AM attachment (graph 50A discharge of FIRST 18AH SLA w/comments showing charge available in 3 areas of charge) BattTest50AchrctrznEX18slaGrph294-2006-1 crp30p.jpg
or PM me and I’ll email it.

Hi Dale,

Thanks for the reference.

Here’s a direct link:



Sealed Lead-Acid batteries have, by their chemical nature, a normal cell voltage (fully-charged) of about 2.1 volts per cell, so a fully-charged battery with 6 cells (nominal 12 V) at rest should show an open-circuit voltage of about 12.65 volts. These values come from the cell chemistry.

To charge any battery, you must impose a voltage higher than the cell voltage. Charging creates a phenomenon commonly called “Surface Charge”, which increases the open circuit voltage somewhat. This additional voltage comes from non-chemical forces, and does not appreciably add to the energy available from the battery during discharge. This surface charge dissipates on its own after several hours, or extracting a small amount of energy from the batter can do this (15 seconds of a ~50 W load should suffice)

Aggressive charging, defined by FIRST as above 6 A charge current (but more commonly agreed to be about 1/10 the Ah capacity, here 1.8 A) does indeed decrease the battery’s lifetime, and of course decreases the time to charge, but does not increase the amount of energy available during discharge at all. The energy available for discharge comes from the mechanical construction of the cell, and does not change with charging.

If you really need a charged battery fast, fine, but it only buys you speed, not energy. Indeed, sensing circuits in battery chargers are more likely to cut off charging early in a high-current charge cycle than on-time, meaning you actually end up with a little less energy.

You see, overcharge is really catastrophic, since the water in the cell is electrolyzed into hydrogen and oxygen gas which are lost from the cell. Once lost, the water (and it’s support of the energy in the battery) is lost forever. A battery charger would better undercharge a little than overcharge a little to avoid dramatic loss of battery capacity.