Voltage: Starting battery vs. Deep Cycle

I have six 12V deep cycle batteries (lead sulphur type) in an electric vehicle (similar to a golf cart). The batteries are new.
While charging the voltage is 13.8 Volts. After a full charge and waiting 2 to 3 hours the battery voltage is 12.3 Volts.
I also measured the voltage of the starting batteries of a number of different cars and found them to be consistent at 12.5 Volts.
Does this mean that the electro-chemistry of a deep cycle battery is different from a starting battery OR are my deep cycle batteries simply not fully charged?
Note: my digital volt meter may be off, however, I think that the difference in voltage is significant.
I appreciate your comments.
Thanks, Hendrik

Deep cycle batteries require a deep cycle charger. Charge voltage need to finish around 15 volts on a deep cycle battery for 6 cells (2.50 volts per cell).
Standard automotive chargers voltage will finish at approximately 14.2 volts. It is a big difference between 14.2 and 15 volts when charging a deep cycle battery.

Trying to jam that last 5% charge into a lead acid battery mostly makes heat and combustible air/fuel mixture.
Electric vehicles are short range and low speed due to the very limited energy storage and high weight.
Add to that if you want the batteries to last. You won't be deep discharging them on a regular basis.

The materials inside both batteries are the same, lead acid, therefore the voltages are the same. Your best bet is to buy a hydrometer to determine when your deep cycle batteries are up to full speed. Depending on how much your Deep cycle batteries are discharged, I've seen 36v golf cart chargers take 10-12 hours, charging at 20 amps, before they shuts off. You need to keep a bottle of distilled water handy and keep the batteries fully charged. I would also recommend getting a good load tester and measuring the output of your batteries. Always keep them fully charged. Never run them flat. If the hydrometer on one cell is lower than the others, you may want to learn how to equalize the cells.

George

The materials inside both batteries are the same, lead acid, therefore the voltages are the same. Your best bet is to buy a hydrometer to determine when your deep cycle batteries are up to full speed. Depending on how much your Deep cycle batteries are discharged, I've seen 36v golf cart chargers take 10-12 hours, charging at 20 amps, before they shuts off. You need to keep a bottle of distilled water handy and keep the batteries fully charged. I would also recommend getting a good load tester and measuring the output of your batteries. Always keep them fully charged. Never run them flat. If the hydrometer on one cell is lower than the others, you may want to learn how to equalize the cells.

George

A true deep-cycle battery is supposed to be charged at a higher voltage then a cranking battery. 15 volts normal charging and 16 volts to "equalize." That's why deep-cycle batteries never get fully charged when hooked to standard automotive alternators.


12.3 volts is a battery with only 70% charge - IF your voltmeter is correct.

You stated it's a 12 volt battery which makes me think it is NOT a true deep cycle - but more of a quasi dual-purpose battery. But, I could be wrong. What brand and what part # batteries are they?

Lead acid, be they deep cycle or not, batteries of 6 cells each 2.1 volts should equal 12.6 volts fully charged subject to temp variations.

A while back I asked on here about good brands of deep cycle chargers, LJD and others offered some opinions and I did some research, the so called smart chargers use an initial bulk charge rate plus an absorption charge and then the more expensive units like Xantrexo have the equalization charge rate of 15 to 16 volts which supposedly allows for removal of any remaining lead sulfate on the plates.

Even if the cheaper are referred to as smart chargers I dont think they have an automatic equalization mode and instead you have to manually set them to a real high 15 to 16 volt range and stop once they outgas excessively otherwise you can damage the plates and/or boil them over.

Schumacher (spelling??) and Xantrex seem good and were recommended by LJD and other here but those are pricey when you get to the higher amp rated units. They have some Xantrex 10 amp on Flea Bay but with my 375 amp hour RV battery bank they recommended at leat a 40 to 50 amp charger and those sizes in a true 3 starge smart charger with Bulk and Absorption and Equalization are EXPENSIVE but so are 3 or 4 true deep cycle batteries also so if you greatly extend their life it may pay off to get a high quality deep cycle charger???

John T

Trojan rates their T-105 or L-16 deep cycle batteries as fully charged when they read 12.73 volts at 80 degrees F (after sitting for an hour without being charged). 12.6 volts is a 90% charge, 12.3 volts is 70%, 12.1 volts is 50%, and 11.5 volts is 10% charged (i.e. 90% dead).

I had a charger once that had two switch positions for charging rate/terminal voltage and one of the positions was for deep cycle. Have no idea as to the chemical content, voltage/charging current requirements, or anything else other than they are made from (fewer for a given size) thick plates that can tolerate deep cycling more frequently and deeper than coventional thickness plates in lead acid batteries.

But don't think they can't pump some amps when they want to. I bought a used tractor a few years ago ( 70 drawbar hp JD 4010D) with a (1) size 27 deep cycle battery that was several years old and I used it for a couple more years at least and that battery could start that tractor year round. I couldn't believe it.

Mark

Thanks LJD.
My batteries are TROJAN 27TMX, Deep Cycle, 12 Volt, 105Ah@20h.
Is this a true deep cycle battery?

Other batteries I was offered are Crown, deep cycle, 27DC115. I have my doubts about the "deep cycle" properties of these. Would you care to comment?

Thanks, George.
My bottle of distilled water is always at hand; I know that keeping the cells filled is important.
I will heed your advice for putting a hydrometer to the battery cells.
Is a load tester necessary or just "nice to have"?
Thanks, Hendrik

Thanks, buickanddeere.
Yes, pumping that last part of charge into the batteries makes them boil (hydrogen/oxygen mixture). I suspect that the efficiency of the kast part of the charging cycle is very low.
Shouldn't deep cycle batteries tolerate deep discharge? Or are they jus a tad less sensitive to deep discharge than the average starting battery?

thanks John for your excursion into chargers and charging processes.
A high tech charger for six 12 V 105 Ah batteries in series will be prohibitively expensive, I'm afraid, if available at all...

Thanks Mark for your comments.
Concur that a deep cycle battery starts an engine at least as fine as a normal starting battery.
From reading all replies I gather that a "smart" charger helps to
a) charge the batteries to a higher degree
b) increase the life of the battery, while retaining the capacity (AmpereHours).

The deep cycle battery has thicker plates and sometimes calcium alloyed with the lead.
True they deep cycle batteries resist deep cycling better than starting batteries but.........It's best to use more batteries and/or a small load for a shorter duration of time. To limit cycling depth.

That being the case, on equal starting capabilities, means to me that the regular starting battery is way over kill for the job at hand......but maybe that is a built in safety reserve.

In my boat I have a deep cycle and a regular starting battery wired in parallel. On a regular starting battery at my ambient temp and altitude, a full charge is 12.75v. On my boat with both in parallel, the DC will not go above 12.6 on full charge and drags the regular batt down to that level. No biggie but a difference and possibly attests to the fact that a DC is fully charged at a lower voltage, hence the reason for the dual voltage battery charger.

Mark