Ratio of charger amps to battery bank AH's?

[bevans6]

Obviously any charger will eventually charge a battery bank, but there should be a ratio for best practice and acceptable charging rates.  I am planning an initial house bank of four 6V 220 AH wet cell golf cart batteries, configured as a 24 volt battery.  What size range of 24 volt charger should I be looking for?  My simplistic thought is that if I discharge to 50% I have to replenish 110 amp hours, so a 50 amp charger would do that in a little over 2 hours, a 25 amp charger in around 5 hours, that sort of thing.  But what do people usually end up with as adequate?  I would usually be on the road or plugged in, the case of discharging that far would be rare (actually hasn't happened yet even once) and I can charge them from the alternator while driving.  95% plus of the time the charger will be maintaining only.  But I am sure that I am missing something...

I am thinking of a charger like this, since it will fit in my existing space.  http://www.bestconverter.com/DLS-27-25-24-Volt-25-Amp-Power-Converter_p_356.html and http://www.bestconverter.com/IQ4-Smart-Controller-1224-Volt_p_321.html

thanks, Brian

[JackConrad]

I recall reading somewhere that your maximum charging rate should be 20% of total battary bank capacity for optimal charging  and maximum battery life. Jack

[Sean]

Brian,

Accepted best practice is to bulk charge flooded batteries at C₂₀/5 (one fifth the capacity in amp-hours at the 20-hour rate).  AGMs can be charged much faster, at C₂₀/2.  Gel cells must be charged at less than C₂₀/5.

So for your 220 AH bank (at 24 volts), you could charge it at 44 amps since they are flooded batteries.  (If they were AGM, you could charge at 110 amps and really cut down your recharge time, one of the key advantages of AGM batteries).

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

(Jack and I were typing at the same time.  Glad we agreed!)

[Sean]

... I have to replenish 110 amp hours, so a 50 amp charger would do that in a little over 2 hours, a 25 amp charger in around 5 hours, ...

Sorry, missed speaking to this part.

While that makes sense mathematically, real batteries don't work that way.  They follow an 80/20 rule -- the first 80% of the charge goes into them in the first 20% of the charge period, and the last 20% takes 80% of the time.

When you deplete 110 AH from your 220 AH bank, a 50-amp charger will put about 75 AH back into the batteries in the first two hours.  The last 35 AH will take at least another two hours.

This is the reason that, for boondocking purposes, the capacity of a battery bank is considered to be 30% of nameplate for flooded batteries (charge from 50% to 80% before the next discharge cycle), and for AGMs it is 60% (charge from 20% to 80%).  Your 220 AH bank will deliver 110 AH the first day you use it after a completely full charge, and about 88 AH each cycle thereafter if you are minimizing the generator run time (running for 2 hours vs. four hours).

-Sean
http://OurOdyssey.BlogSpot.com

[Don4107]

WOW, there is a ton of basic info in this thread.  Even a dummy like me can gleen enough info to select the type of battery best suited to my needs, the charger needed to maintain them, the best ways to charge them to keep them alive and happy, and how to minimize gen run time.  The only other thing I need is a state of charge meter and even a goof like me should get a few good years out of the batts..

Great stuff!

Don 4107

[Lin]

Although my guess would be the figure would come up about as Sean said, the manufacturer of the AGM batteries I have specifies things like max charge amperage, bulk charge voltage, and float charge voltage.  If ever in doubt, I always check with tech support.  Most times they are helpful and really know what they are talking about.  In some problems, one sentence from a competent person can save you days of pondering and experimentation.

[TomC]

Course the easiest thing to do is just to run the generator (when boondocking) until the float light comes on the inverter.  Works for me.  Good Luck, TomC

[Sean]

Course the easiest thing to do is just to run the generator (when boondocking) until the float light comes on the inverter.  Works for me.  Good Luck, TomC

Tom, if you have a three-stage charger, and the absorption time is set "properly" for a complete charge (two or more hours), then waiting until the Float indicator comes on means you are getting very little charge out of the last hour or so.

By the time the charger has finished the bulk stage, charge current is at the upper limit.  However, it ramps down nearly linearly to the amount permitted by float voltage over the absorption period.  So let's say the charger cranks out 100 amps in bulk, but only 10 amps in float.  After an hour on the absorption timer, current is just 55 amps or so.

For every hour you run the genny during bulk, you would be putting 100 amp-hours into the batteries.  But over the duration of the absorption period, you are only putting in an average of 50 amp-hours per hour.

What we do when we are boondocking is to set the absorption period down to either 20 or 30 minutes, whereas on shore power we have it set at three hours.  This gives us the most bang for the buck on every hour of run time.

It may not sound like much up front, but over time that saves us 40-50% off our generator bill.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[TomC]

Sean-Good going-I'll give it a shot-thanks!  Good Luck, TomC

[stevet903]

Lots of good reading on this site:

http://www.smartgauge.co.uk/chargesize.html

Steve