Sanity check needed on short-term house bank configuration

[rhumbunctious]

I need to set up a higher capacity house bank for several months, and have at my disposal the following 12V batteries:

2x 110 AH AGM (same type/age)
2x 110 AH Flooded Deep Cycle (same type/age)
1x 95 AH Flooded Deep Cycle

While I know it's common opinion that it's most optimal having all batteries in a given house bank be of the same type and age, I'm assuming that having all of the above in the same bank will be workable for a few months without major problems.

The single 95 AH battery will get worked harder, depleted further each charge cycle, etc. but I'm considering that particular one "disposable". If it lasts 3-5 months, I will be happy to have the little bit of extra capacity in the bank. The two types of 110 AH batteries should get along fine together as they have nearly identical specs as far as charge rates, voltage, etc.

I will be charging the bank with a Sterling 30A smart charger and recharging when the bank gets down to 80%.

So.... what am I missing?

[Minnewaska]

I'm not the electrical engineer on the board, but believe each diff batt you spec'd would require different charge rates. It just doesn't makes intuitive sense that your charger would know when to stop. Would it stop when they are all max'd, which probably cooked the first one's to get there?

So my ill informed reply..... yes, you're insane.

[BubbleheadMd]

You can do it, but it's best if you charge the "odd man out" on it's own charger.

[sailingdog]

One issue is that by only going down to 80% capacity, you're actually going to be wasting a lot of the capacity of the bank. If that is your plan, you're probably better off not combining the various different batteries, and using either the AGM pair or the wetcell pair as the house bank and running it down to the 50% mark instead--or using both pairs as two different house banks and alternating between them.

Having the 95 AH battery in the mix is pretty stupid IMHO.

Mixing the AGM with the wetcell is also a bad idea, since the AGMs have a much higher current acceptance rate when in bulk charging mode than the wet cells do.

Also, if you combine all the batteries for a bank size of over 500 amp-hours, the charger you have is woefully undersized to deal with it.

[Minnewaska]

Won't the 110ah AGM and wet cells discharge and recharge at different rates too? How could one charger deal with the hodgepodge, without hurting them all?

[jrd22]

Since you are only going to do this for several months I don't see any problem. I would think that if you set your charger for the lower rate so you don't damage the wet cells you should be fine, not optimal but this is temporary.

[LakeSuperiorGeezer]

AGM batteries can be charged with the same charger that is used with regular flooded cell batteries (those that have caps for adding water). Do not confuse an AGM battery with a gel cell battery, which really can be destroyed easily by improper charging. The AGM can be charged a higher rate than the regular batteries, they self discharge at a lower rate (1% to 3% per month) so have a longer shelf life and will not spill acid. I have seen claims where they can be mounted in any position. Any hydrogen and oxygen created in charging is recombined into water so AGM batteries are sealed, except for venting if overcharged at high voltage. The disadvantage with these batteries is the cost, which is about twice that of a premium flooded cell battery. As with any battery an intelligent three-step charger should be used. The first step brings the battery to 80% charge at 14.4 volts with a limit of 10% of the ampere-hour rating. For instance if you have a group 27 battery with a 100 ampere hour capacity, you should charge at 10 amps. The next step is to bring the battery to 100% charge by keeping the voltage at the same level as the first step, 14.4 volts, which means the amps going into the battery have to decrease. The final step is to float the battery at 13.4 volts for long-term maintenance, just leave the battery charger connected all the time. Note that the voltages specified above will change depending on the temperature so an intelligent battery charger should also be temperature compensated. Since the voltages are the same for the flooded cell and AGM battery, there should be no problem connecting the battery banks and charging with one charger. The larger batteries and/or the ones needing more charge will take the amps because they have a lower voltage (actually a lower internal resistance) until reaching full charge. Other batteries already charged will be OK because the 14.4 volts will not be exceeded and oxidation of battery plates will not be an issue on the charged batteries for the short length of time needed to bring the other batteries to full charge. However, the really important thing is to have a marine charger with separate windings so there is no electrolysis. I have other postings on batteries concerning electrolysis: http://www.sailnet.com/forums/cruisi...0hz-areas.html

[mitiempo]

I agree with LakeSuperior for the most part. The exceptions being the 10% limit. Flooded can and should be charged at about 25% of their amp hour capacity (at the 20 hour rate). AGM batteries have much lower internal resistance so can be charged at a higher rate.
The charger should be a marine 3 stage charger. The boat's AC earth (green wire) should be connected to the 12 volt system's ground point for safety (ABYC reg as well). The best way to eliminate electrolysis is with a galvanic isolator.

[newhaul]

my question is why do you need the high bank capacity if you are bringing down only to 80% then recharge leads me to believe you are going to be dockside most of the time if not the case i am in agreement with making two seporate banks using the agm's in one and the wet cells in the other and alternating and keep the 95 in reserve for the oh s#$t battery.

[mitiempo]

Taking a battery to 50% state of charge is normal before charging. 80% makes no sense. AGM and flooded are almost identical in charging voltage so one bank is workable.