why not to pull 12volts off mid string

[mobile_bob]

there is often a question asked here and elsewhere in re to drawing 12volts off of a 24, 48 or more string
and the advise is never to do so under any conditions

here is a practical application of the "law" in operation

in my shop/lab/garage, i have a set of 6 agm 200amp/hr 6volt batteries, i got them as blems from my battery supplier
and have 4 setup in series for 24, and the other two in series for 12volt testing

somehow i ended up killing the 12 volt pair finding them flat dead one morning, ouch!

now i have 24 and 48volt charging ability, and 12volt only when the trigen is being tested via 25ft jumper cables
so i set out to get the pair charged back up asap on the trigen

problem was i needed many hours of run time to get them back up again, given the jumper cables drop the voltage
to about 13.4 under load, which is painfully anemic when it comes to fast charging anything. nonetheless i got them up
to 11.9volts which is still effectively dead dead dead.

my 12volt charger is at friends house some 50miles away, and it is one of those stone age monsters that will burn up a battery
unless you babysit it carefully.

next i decided to insert them into a string with a pair our of the 24v bank so that i could let them charge via one of the inverters overnight.

i get up this next day to find the good pair sitting at 7.4volts each and the poor pair at 6.05volts, damn thats hard on the good pair
but because it is cold in the shop they seemed to handle the overcharge without venting.

i then decided to go get a new scheumacher charger, and charge the bad pair only until the came up to near full charge.
and also used a load to knock the good pair down.

after getting both pairs within .3volts of each other (on a 12volt basis) i then applied a load to the set via the inverter 750watts for an hour
and then reset the inverter for charging and walked away.

came back about 12 hours later, inverter shut down because of full charge, and noted there was now a .2volt difference.

doing the same loading, recharging the bad pair seperate to get it closer, wash rinse and repeat.

i will now check this morning to see if i finally got them back in balance.

moral of the story?

never ever draw 12volts off of a 24, 48 or other string of batteries, the result will be an imbalanced set
which will lead to overcharge of the good batteries and an undercharged battery you are drawing from.

this is likely one of several very good ways of dramatically shortening the life of your batteries, and
once found take many hours of effort to make right again.

in my case it didn't start out as a draw mid string, but a seperate pair making the 12volts
but the resulting efforts to get them recharged clearly illustrate the problems associated with imbalanced loading and charging.

bob g

[BruceM]

Good illustrative story, Bob.

I expect those batteries will be the "weak sisters" of that set forever more, and only frequent equalization will keep the set alive.  Once the charge/discharge impedance of a battery is off compared to the others, every charge/discharge cycle makes things worse. (Over and under charged batteries in the string until failure by plate erosion or sulphation.)

That's why I like the individual shunt regulator system developed by Manzanita Micro for EV battery (large series string) charging.  http://www.manzanitamicro.com/index.php?option=com_virtuemart&page=shop.browse&category_id=17&Itemid=64 

I have some  experience with  my own version of this charging system on an experimental, ten 12V battery string of cheap marine 110AH "deep cycle" wet lead acid batteries. (120VDC)   Without any  equalizing, and just holding the absorption phase voltage for a few minutes extra at the end of each charge cycle (sort of a micro equalization), the batteries' individual variation in charging time drops dramatically in must a few charge/discharge cycles.  By logging charge variation times, I'll know which battery might need some attention or replacement. Replacing one bad battery in an otherwise still good string CAN be done with this system, with a small penalty in charge efficiency. 

I'm now trying to finish my software for automating the PV and AC generator charging, with SOC calculated by real time current inputs.  Today I also mess with checkout of my 120VDC output regulator.  Progress is slow, but recent warmer weather and sunshine seems to help.

[mobile_bob]

just about 3 hours away from the 4 cycle on the set, the last cycle remained about 0.1 volt apart

it may be that will be as good as it gets, the weaker pair still make it to 12.8 which should be fully charged
while the better pair settle over 13volts.

its just an eye opener to say the least, and i knew better about such things having been preaching about it
for years.

it was just interesting to see just how difficult it can be to get the weaker batteries back up to a reasonable condition
and serves as a reminder to me about just how critical it can be to allow things to get out of hand.

i can also see the need for a separate charging source sized to charge a single battery of a string, in my case that battery will
likely end up being either a 4volt or a 6volt if i go flooded cells, and 12volt if i go with AGM
the bank will be 48volts nominal in any event, so i want the ability to care for a weak sister in the string without overstressing the
other batteries with a long unnecessary equalization charge.

bob g

[BruceM]

Bob,
Take a good look at the Manzanitamicro.com regulators, even for just a 48V string I think they will make a big difference in battery service life.  It's like having a separate charger on every battery, for every charge cycle.  It was originally developed for AGM batteries- they don't tolerate frequent extended equalization charges which the industrial deep cycle batteries will.  If you are designing your own charger anyway, it would be easy enough incorporate the individual battery regulators into your design.  Their analog regulators are $45 each.  Your charge control system would need to watch just the two analog values- the highest shunt temperature and the lowest to determine when to switch to low current charging, and when the charge is complete.  (I like to also monitor charge current for better charge completion detection.)

The EV folks originally had no choice in batteries- they needed the AGMs for their much faster charge/discharge rate.

I was hoping for a set of AGM's, but prices went up too much for me.  Their higher charge acceptance rate, and higher efficiency makes them very appealing.  Maybe for my second set of batteries, after this on grid house is sold and I get out of debt. 

Bruce

[Jedon]

How about drawing 12V off mid string but only very low power? I'm thinking 60W max for POE ( power over ethernet ) mostly to power some LED candle lights. So not more than 5A @ 12V, off a 105AH 48V bank ( 8 6V golf cart batteries ). I suppose running that 12V over 300ft from the batteries to the house would incur more losses than using a transformer at the house?

[BruceM]

No, even a very small load on for long periods will cause problems, Jedon.  Just don't do it. Without something like the Manzanitamicro battery shunt regulators, you have no way to even the charge without almost daily overcharge (equalization).

Using a DC-DC converter to get the 5 or 12V or whatever voltage you need from your 48VDC at the house is the most efficient solution.

It is  inefficient to go through an inverter, then to a AC-DC supply, but if your inverter is on all the time anyway, it is the easiest solution.  Your load will probably be less than the idle current (no load) of the inverter.

[mobile_bob]

i agree completely

anything more than a few tenths of an amp, intermittent in nature, is going to cause an issue sooner or later.

5amps will certainly show up as an imbalanced string far sooner than you would like.

bob g

[Jedon]

Okay I guess I'll just use a transformer at the house, both inverters are on all the time anyway along with the fridge and freezer and DVR etc so it will probably just be an unmeasurable load.

[LowGear]

You guys have really got me spooked.  Here is an alternative story on another thread in this site.

Over in "12 Volt Inverter Meets 36 Volt Golf Car" Post #31 by Cujet

FWIW, I have a similar setup. However, I use my 48V golf cart at 24V to start aircraft engines and run the avionics. I simply hooked up to 3 of the 8V batteries.

I had intended on swapping the hookup, but never found a need. I have 2 of these same "club car" carts. The batteries on the one with the starting cable outlasts the other by years. This is because I rarely use the other cart (it's at the little airport). The "start cart" gets daily use and my current set of batteries are 8 years old and very strong.

Casey

[mike90045]

You guys have really got me spooked.  Here is an alternative story on another thread in this site.

Over in "12 Volt Inverter Meets 36 Volt Golf Car" Post #31 by Cujet

FWIW, I have a similar setup. However, I use my 48V golf cart at 24V to start aircraft engines and run the avionics. I simply hooked up to 3 of the 8V batteries.

I had intended on swapping the hookup, but never found a need. I have 2 of these same "club car" carts. The batteries on the one with the starting cable outlasts the other by years. This is because I rarely use the other cart (it's at the little airport). The "start cart" gets daily use and my current set of batteries are 8 years old and very strong.

Casey

I don't know what to think. theory says you will kill the tapped battery.  But if your practice shows otherwise, go for it.   Personally, I'll adapt something before I tap a battery

[mobile_bob]

why not just throw another 12volt battery onboard to power the inverter, and when you get back in to plug in the carts 36volt bank
you also plug in a 12volt charger for the added battery?

bob g

[LowGear]

Hi Bob,

Your idea is sensible and easy but those qualities aren't what makes Camp Aloha wonderful.   The inverter is on the shelf until the ATV hits the road with some reliability.  I want to use the three batteries in parallel to provide lots of good 12 volts.  Let's not forget that storage space on an ATV is just a little tight as well. 

Casey

[mobile_bob]

if you use three in parallel, then you have no issues with charge balance
i thought you were back to wanting to pull off 12volts from the 36volt string.

bob g