# Battery charging question



## Coley (Jul 26, 2007)

I realize you should not have old batteries along with new ones in the pack, but.

While charging them up (72 volt, 20 amp.) charger, I have a real difference in voltages on all 6 batteries.

The newest is showing 14+ volts and warmer than the rest.
One of the oldest is showing not much on the hydrometer, but 12.5 volts and holding well on the load test. Also warmer than the rest.

The rest are from 13.1 to 12.9. Equally slightly warm.
Any suggestions as to how to correct this or explain what is happening.

These are marine, deep cycle batteries.


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## Mr. Sharkey (Jul 26, 2007)

You answered your own question:



Coley said:


> I realize you should not have old batteries along with new ones in the pack


That's _exactly_ the reason you don't mix-n-match batteries in series strings.

You have few options, aside from the obvious proper solution of purchasing all new batteries. One would be to install battery management controllers, an expensive proposition at best. The other would be to do what KiwiEV has done and have a separate, regulated charger for each battery in the pack (or for sub-sections of the pack, arranged by age).

If you think it's bad now, wait a thousand miles or so, the discrepancy gets worse with each charge cycle.


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## Coley (Jul 26, 2007)

Thanks, I didn't think an older battery, that has been working well, would make a new one overcharge.


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## KiwiEV (Jul 26, 2007)

Gosh that's spooky how one battery can affect another. Coincidently I now have that faulty battery in my pack replaced. I've only done a total of about 5 miles of (not legal) test-driving so far with charging in-between so the pack won't know any difference. The car's in pieces again while I fix glitches and rewire stuff anyway and won't be moving for a fortnight at this rate (until my new recharging plug & socket arrive).

I'm sorry Coley, I just realised I hijacked your thread. It was a very handy one though so thank you.


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## Mr. Sharkey (Jul 26, 2007)

If you are interested in the theory that's causing this, consider the following:

Let's make it simple, two batteries in series, one new, one old. Discharge the pair. Not an optimal situation, but since the discharge current is equal to each battery, they both deliver the same numer of ampere hours. Don't go past the capacity of the weak battery and you will have two discharged batteries ready for filling (go past the capacity of the weak battery and you risk reversing it's polarity, and finishing it off for good).

Time to charge up. The new battery is more efficient, and charges quickly, the old one is a bit creaky, and takes it's time.

In a normal situation, the two batteries would have nearly identical recharge times, and would reach full at about the same voltage. Their internal resistance would rise, terminal voltage would rise, and the charger, being set for a maximum finish voltage, would stop pumping current through them as it "tops out"

Now consider the two mismatched batteries. The new one reaches "full", and it's terminal resistance and voltage rise. The old battery still has a low resistance, and low terminal voltage. The charger, looking at the two batteries in series can't tell that one's done and the other isn't, all it knows is that there's still a load out there (the combined resistances of the two mismatched batteries being less than two good ones), and it keeps pumping current through them, attempting to reach the finished voltage.

The old battery sees reduced current due to the higher resistance of the new battery being in series. It's charge rate actually reduces.

The new battery, however, is being forced to conduct current it has no use for. It's response is to hydrolize (gas) and heat up.

A battery management system would allow some of the charge current to bypass the new battery when it gets full, and still allow the old battery to get current it needs to finish. You would still not have an optimal system, but at least you wouldn't be frying a new battery to death while attmpting to keep an old one on life support.

Similarly, using a portable charger to finish the old battery individually when the new battery is done is bothersome, but would allow you to charge the full pack without eventually eroding the plates in the new battery from excessive gassing.


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## Coley (Jul 26, 2007)

Thanks Mr. Sharkey.
I think I get the idea as to why the new one is getting gaseous.
Thanks


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## nutsandvoltsvw (Oct 29, 2007)

Thanks for the battery knowledge...

Found this charger in the marine and boat parts catalog

If 4 amps per battery is enough to charge deep cycle batteries in 16 hours than at around $200 each I would need 3 of them for my 120 volt system to charge each battery individually and the two aux batteries I am planning......around $600 is better than some of the 120 volt chargers that do the entire string at once... Is this better or am I missing something REAL important??? This sounds like a pretty smart charger...

Would this help the mismatched battery problem?

Robert
nuts and volts

[h2]12/24/36 Volt 4 amp 3 bank marine grade waterproof charger.[/h2]
This Dual Pro Lil Pro 3 marine battery charger is a newer entry in the Dual Pro line of quality marine battery chargers. The waterproof Lil Pro chargers are 12 volt independent output, 4 amp per bank, temperature compensating battery chargers. These independent bank battery chargers have a separate positive and negative connector for each bank, and a separate microprocessor chip which controls the charging algorithm of its own bank. The batteries charged may be of differing size, age, state of charge, or type (sealed, flooded). This works well for applications like boats with a starting battery and a trolling motor setup. It also works for series strings of three 12 volt batteries in a 36 volt series. After each battery has its own connection, the charger does not care if they are parallel connected for higher amps, or series connected for higher voltages. This is especially useful in higher voltage situations where one of the 12 volt batteries in the string is feeding some 12 volt loads (like lights, fish finders, etc.), and the batteries end up imbalanced. A straight output 36 volt charger used for this situation would not properly charge the batteries and rectify the imbalance, decreasing battery performance and life. 
The Lil Pro 3 three output 4 amp battery charger is appropriate for 12 volt, or 12/24 volt, or 36 volt applications (three 12 volt batteries in series) where an exterior grade battery charger is required or desired. They have flanges for mounting, and will survive water, vibration, and temperature. They are used in marine applications, like boats with a starting battery and a 24 volt trolling motor, or three 12 volt batteries in parallel. When used for a trolling motor application, we use this charger for batteries up to about 50 amp hours. The 80 to 110 amp hour setups should use a 10 amp per bank or larger charger. The Lil Pro 3 can be used as a maintenance charger for larger capacity batteries or systems.
The Dual Pro chargers shut off completely when the battery is fully charged, and the microprocessor monitors the system voltage, initiating another charge sequence if appropriate. Some battery engineers prefer this charging method to decrease water loss. 
Built in the USA.


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## Mr. Sharkey (Jul 26, 2007)

From the description, it seems that charger (or several of them) would work for charging up your traction pack.

I do have concerns when I read: "The Dual Pro chargers shut off completely when the battery is fully charged, and the microprocessor monitors the system voltage, initiating another charge sequence if appropriate. Some battery engineers prefer this charging method to decrease water loss." And: "The batteries charged may be of differing ... type (sealed, flooded)".

Many (if not most) of these type of "automatic" battery chargers are designed for complete idiots who would put their boat in storage, leaving the battery connected to the charger and plugged in, charging until the next fishing season. The charger is designed around the premise that it will have lots of time to provide the battery with the necessary finish charge, and that topping up the battery in a quick and timely manner will not be necessary.

Likely, the charger is designed to end the charging cycle when the battery terminal reaches 13.5+ volts, which is far short of a complete bulk charge cycle, and ignores the requirement for an absorption cycle.

Here's a copy-and-past from an article I wrote for and RV magazine concerning deep cycle battery charging:



Me said:


> Multiple stage charging is the best and most economical way to fill up batteries. Three stages are usually preferred, bulk, absorption, and float.
> 
> Bulk charging is done to return 75% of the energy removed from the battery quickly. Amperage is kept as high as the battery and charging source will allow, usually equal to a value of 10-20% of the batteries ampere-hour capacity ( a 220 ampere-hour battery would be charged at 22 to 44 amps, expressed as C10 or C5. The equation for determining this value is Capacity divided by charging current determines time to charge fully [C value] 220/5=44=C5) When the battery voltage reaches approximately 2.41 volts per cell (14.5 volts for a 12 volt battery), absorption stage is initiated.
> 
> ...


Batteries that are frequently cycled simply must have the complete bulk -and- absorption cycles to be expected to have a long life. Anything less is putting the battery back into service without a full charge and will result in eventual failure due to sulfation.

Additionally, since the charge profiles for wet and sealed batteries is different, chargers that will charge either "interchangeably" usually error on the side of caution (towards the less aggressive charge cycle for sealed batteries), and will cause wet lead acid cells even more harm.

Two rules of thumb about wet cells:

1) It's _always_ better to overcharge than undercharge

2) If you're not having to add water occasionally, you're not overcharging

Finally, you will have to consider what you are going to do to equalize charge your batteries if the charger isn't capable of providing that upon demand. Equalizing charge is a purposeful controlled overcharging of the batteries periodically to bring all of the cells in the pack to a fully charged state, forcing them to gas fairly violently. This helps to even out any inconsistencies in the cell's capacity and terminal voltage, and helps to destratify the electrolyte. It can be done with a stand-alone charger, one battery at a time, but that's rather time consuming, and while it's happening, and until it's completed on all of the batteries in the pack, you don't want to discharge the pack (drive the car anywhere).

Charging batteries is much more complicated than discharging them.


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## nutsandvoltsvw (Oct 29, 2007)

Thank you for the charging course...


Do you have any suggestions for a 12 volt charger that will do the three stages, that one could buy 12 of, to charge and manage 12 batteries individually? It seems to be a better way to control the charging of the traction pack than doing the entire string at once in series at 120 volts...

Thanks,

Robert
nutsandvoltsvw


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## elevatorguy (Jul 26, 2007)

FWIW, I went with the 4 bank minn kota mk440 which is a 3 stage charger for my atv. I am not sure how this would work on flooded batteries as it may not charge hard enough to gas them properly. It is "universal" agm/flooded type and I haven't tried it on flooded cells. 
My thinking was each battery would have seperate charge and would last longer, I have already noticed that some of the batteries charge at different rates and finish later. 
Jerry

http://www.austinev.org/evalbum/preview.php?vid=1174


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