Starting/reserve battery size

[Ken Juul]

If you go with the jump start, remember to charge it about once a month.  I have a cheap one similiar to the harbor freight, sitting on the garage floor it seems to last about 3 weeks before it starts losing it's charge.  Maybe I got  a bad one, but if it is your insurance policy make sure it is always ready.

[mark_53]

Good point about charging it once a month per manufacturers recommendation.  I've found I can let it go several months and the built in tester still says its good.  I charge it anyway, takes about 5 min until charging stops so it seems to be holding it's charge pretty well. It's less that a year old so I'll have to see how it holds up.

[Footloose]

When it stops holding charge, take it apart and replace the battery inside.  Generally an screwdriver is all that is needed to open the case!

[lazybone]

I use a U1 AGM as my starting battery on Gorbash.  It's located just under the small compartment cover in the aft cabin where I can reach it easily.  It has a high current in-line switch installed so I can connect it or disconnect it easily by feel.  It's been maybe a dozen years since I installed it so I'd have to take a look to remind myself of the details but basically it's wired to the starter with a short run of very heavy wire.  It's connected to my house bank of 4 golf cart batteries through an automatic combiner.

The boat is in western Washington so we never have to deal with extreme cold but I always use the small battery to start the engine and it has always started easily with the exception discussed below.  

The reason I used an AGM battery was so I could mount the U1 on it's side to fit comfortably below the wooden cover including being strapped down inside a standard plastic battery box.  This, of course presents the issue of what battery type settings to use on the TruPower charger and Balmer regulator for my mixed battery type situation.  I can't remember here sitting at home whether I chose AGM or flooded but I know it was whichever one was least likely to overcharge the AGM.  I think I remember that this left the house bank floating at a slightly lower voltage than ideal but it was by only a small amount. The current house bank batteries are about six years old and are due for a capacity check but last summer on a month long trip they seemed to take about the normal amount of time to get down to 50% - so it seems this slight undercharge is not hurting anything.

The problem that has occurred is that charging regime for the house bank appears to be unduly hard on the much smaller AGM even though all the stage voltages on the chargers are optimized for the AGM.  Early on I left the AGM connected to the combiner all the time and found that the first U1 AGM failed to hold a charge after three years.  Another died the same way about four years later.  The third AGM U1 is still going strong after another four or five years.  I now manually disconnect the U1 when we leave the boat for long periods or when the house bank is being recharged after a deep discharge.

I thought about replacing my four flooded house bank batteries with AGMs with all their associated advantages (and expense) but concluded that the source of the U1 failures was more likely from being charged in a mixed SIZE environment than from a mixed TYPE environment.  This conclusion came from watching my Link type battery monitor and noticing the large amount of time the U1 was facing unduly high voltages while the house bank was being charged from a deep discharge.  Of course this occurs at the end of the charging cycle for the house bank but still, the U1 is seeing 14 volts for what I think is way too long given that it is hardly used in starting the engine and so is almost fully charged during the entire house bank charging cycle.  Thus my habit of disconnecting the U1 except when needed.

This theory may in fact by haybag but the system works well enough that it hasn't been worth it to me to rework it.  I have fiddled with trying to get the smart combiner to use it's overvoltage protection circuit to cut off the "combine" when the voltage rises beyond 13.8 volts or so but that induced other problems.  As we're planning multiple month trips up the BC coast now, I will probably replace the house bank "just cause" and may consider AGMs for other reasons.  I would still be leery of the capacity mismatch and would probably still disconnect the U1 much of the time.

Your inputs are welcome!

Randy

Re; combining flooded batteries or banks of different capacities

"since the combiner just places batteries in parallel, they are all at the same voltage. The charge level of each battery is a function of voltage, so they will all receive the same level of charge. The actual charge capacity is governed by their size, age, chemistry and condition."

Copied from the Yardina site.
Tons of great info on his site.