Battery Bank Charging

[squarebob]

If I wanted to charge 8   -   6v 220Ah batteries, set up as a 24v bank, would this be a good alternator to use. I can pick it up for about $125.00

Thanks
Bob

[Ronmar]

Well 8 220AH batts in pairs to make a 24V bank means a 440AH bank?  That alternator is rated at 75A, so that means a 6 hour recharge time give or take other factors, if you spin it at an acceptable RPM.   Does that work for you?

[elnav]

6hr to recharge a 440 A-H bank is perhaps  overly optimistic.

The stated  75 amps  is a cold rating if I'm not mistaken. If the  alternator  remains controlled by  the internal single stage  regulator th eoutput current will steadily decrease as the battery  becomes charged.
If the alternator  is controlled by a external 3 stage regulator  it will take  4.5 hours  to reach the transition stage from bulk to absorption. At that point  the battery will still be short 110 amp hours  but the charge current will decay in an exponential manner  at an ever reducing rate.  However  it will take more than  1.5 hours to reacg a state of full charge. Given the usual  efficiencies of  the charging system it will take at leat 110% to fully  recharge  the 440A-H battery bank if fully discharged. The alternator will therefore have to deliver 480 amp hours  not 440.
Prudence  suggest  batteries not be fully discharged.  however  when you recharge  a partially discharged battery  the acceptance rate is lower  and  in turn this  means a lower total current. 
Running any alternator at full maximum current is going to produce heat which in turn causes a reduced output  and thus longer charge times.
For a charging application involving maximum  output for many hours  you should only use a smart regulator that not only monitors  battery temperature but also  alternator temperature. This will prevent  damage to the alternator  stator windings.
Balmar is one such product  that includes monitoring  alt. case temps but also has an Amp Manager.  This feature permits  limiting the maximum output current  as well as invoking  additional  safety features.  This will prevent heat damage.
 

[squarebob]

The unit does not have a regulator so an external unit would be required. Balmar seems to be the unit of choice. My best guess base figures say I will consume about 5Kwh per day.

5*42Ah (1Kwh/24v= 42Ah) = 210Ah or about 50% DoD. Right on the money. 75A at 80% efficiency would be 60A. 210/60 = 3.5 hours for a FULL recharge as best as I can figure. Longer than I want to run the gen. I guess I need to look for a 150Amp unit to get charge times down to a more realistic (in my mind) run time. Every time I turn around, I realize I have missed something here or there in trying to come up with a workable solution. On the bright side, I just found out my friend has 200 gallons of WMO he has collected over the years and I can have it.

Bob

[mobile_bob]

start with what your daily needs are, for instance lets say it is 300amp hours

divide that by 3, and then multiply the result by 10

therefore,  300/3=100   and 100 x 10= 1000

1000amp/hours will be what you want the battery bank sized at, because

the most efficient charging, and best return on investment on batteries, all things considered
is to run them from 50-80% State of Charge.

between 50 and 80% the batteries can take a rather large charge rate, and charge much faster, but
they still aren't fully charged,  so periodically you will have to do a complete charge.

figure on doing that every week to 10days, and thing will be fine
going from 80-100% is going to take many hours of run time, so you don't want to do that every day

going back to our example a 1000amp battery bank can handle a couple hundred amps charge rate from
50-80% state of charge, no problem, so

putting back the 300amp hours each day would take about 1.5 hours of run time if you can deliver 200amps to
the example battery bank.

so in practice monday through saturday you can get by with approximately and hour and a half of run/charge time
and on sunday you will probably run 8 or 10 hours at a low rate to top them off, but the sunday long runtime need not be
a problem, because you will have a ton of ampacity that is not being used.

that extra capacity can be used to cover much of the normal loads, plus you can schedule to take advantage of the extra capacity
but scheduling wash day, or some other activity that would make use of the extra capacity.

if you follow this routine and religiously follow the weekly full charge then the normal sulfation will not have time to harden and crystallize
and will be broken down fully and the batteries will be fully restored, however  doing periodic equalization and logging the health of your cells with
a hydrometer is always a good idea.

ymmv, and others will likely have other thoughts on the topic
just bear in mind that short cycling greatly increases battery life, but it always require a dramatic increase in engine run time to recharge them.
so in the end you get more cycles, but at a huge expense in fuel consumed
also going using more than 50% of the battery capacity on a regular basis will dramatically shorten the battery lifespan

50-80 regime has proven to be the most efficient and provides the best return on investment of any charging regimes i am aware of for
flooded lead acid batteries.

bob g

[squarebob]

Thanks for the info.
When in the bulk charge mode, ( 24V bank ) what would be the optimum voltage to set the regulator at. A Balmar is preset at 29.2V for deep cycle batteries. It can be changed, but is there a "best voltage"..............

Bob

[mobile_bob]

the proper voltage set point depends on two main factors,

one being battery technology, flooded/agm/gel?

the other is ambeint temperature and battery temperature.

generally 28.8 vdc for a flooded cell at 77 degree F is about right for bulk and absorption
and maybe 27.2 for float.

a temp sensor mounted to one of the battery negative posts, somewhere in the middle of the cluster
will allow the regulator to compensate for temperature, and also protect the batteries from overheating.

another sensor mounted to the alternator case is used to monitor the alternator temperature and protect it
from dangerous overheating as well.

Balmar publishes a lot of info you might find helpful

others like Ample Power also build regulators and have published a lot of useful info as it relates to setting up the regulator
to get the batteries fully charged as efficiently and safely as possible.

bob g