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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #1  
Old 04-12-2012
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Gel battery weakness

As my other thread mentions, I didn't even realize I had Gels and plan to swap to AGMs, which is what I thought I originally had.

Nothing is easy. Bigger alternator, new external regulator with temperature sensors, premium AGMs...... and it's looking like a $3k house bank job with labor. That was not my plan!

It's been time consuming to just figure out what's in there now! I have a hard time believing there is no external regulator, even with the Gels, but I can't find one.

While I didn't really want Gels, obviously they lasted 8 years and I've slapped myself on the forehead and am considering just dropping in replacement Gels myself. Same wiring and presumable the system is properly set up for them.

What brand 12v (group 31 sized) Gel batteries would you recommend, if I do this?
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Old 04-12-2012
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Re: Gel battery weakness

I guess you have been very lucky unless the Gel come standard with the boat and they have already the alternator regulated for them . The bigger problem with gels is its sensitivity to the charging max voltage that has to be slightly lower than on normal batteries. A normal alternator set up would risk to ruin gel batteries fast. For AGM batteries the voltage is slightly different.

Regards

Paulo

Last edited by PCP; 04-12-2012 at 03:26 PM.
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Re: Gel battery weakness

I finally got a copy of the wiring diagrams. It shows what is referred to as a "shunt" between the alternator and the house battery. Don't know where it is, but suspect it knocks down the voltage.

Makes some sense that this was installed for the original Gels. Also assume it needs to come out if I went to AGMs.

All the more reason to just swap out the Gels and stick with them.

So, whose should I buy?
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Old 04-12-2012
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Re: Gel battery weakness

The shunt is used to measure the amperage flowing through the system - not for regulating voltage. It's essentially a flow meter.

An example:
DC Current Shunt 500A/50mV - PN 8255 - Blue Sea Systems
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Re: Gel battery weakness

mine-
A shunt should not reduice the voltage any. A shunt, in battery charging systems, is just a very precisely machined resistor with a very low resistance. It is placed inline with the alternator (either in the hot or ground side, depending on philosophy) and then a very sensitive voltmeter is connected to the two sides of the shunt.
That voltmeter just measure the voltage consumed by (lost in) the shunt, typically in millivolts. When the shunt and voltmeter have been matched, they are called an ammeter, because the amount of amperage flowing through the shunt (and therefore, through your charging system) can indded be measured by the amount of millivolts consumed by that shunt.
So if you have an ammeter on the boat, or a "battery monitor" of some kind, there normally will be two small wires, in a twisted pair, that go from the shunt to the "ammeter" or monitor. And then two more wires that actually supply power to the ammeter or monitor.

Typically, for every 50 or 75 millivolts that a shunt consumes, one amp is flowing through it.

For smaller systems, a 20-60 amp "ammeter" is designed with the shunt actually internal in the meter, and the whole system output is run through the meter directly.

A shunt kinda looks like a six inch long piece of bus bar, with two extra screw terminals set in from the ends, and one cable lug/bolt at each end. And in theory, there's a plastic cover over it so you can't make big sparks. (That's one reason they are sometimes put in the ground side of a system, less chance of making the big zap.)

As to whose battery to buy...No really, are there are battery DISTRIBUTORS near you? As opposed to retail stores that have to mark 'em up high? Or a Sams' Club?
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Re: Gel battery weakness

Thanks for the shunt description. I guess it's still a mystery why the alternator hasn't fried the Gels over time. There is also a "charge divider", which accepts input from the alternator and outputs to the start batt, starter and house batts.

My local battery distributor wants to sell me AGMs. Odyssey in particular. I don't think they ever sell Gels. The specifically called them "yesterday's technology".

I would really like to source the best quality Gels and just have them ground delivered to the house, so I can swap the out.
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Re: Gel battery weakness

Best quality, who knows. Maybe Sonnenscheisen (did I just curse in german?) or however they spell it, they've at least been known for endorsing the technology and have a good name, although I think they've got a US partner these days as well.

Or the Chinese, ThunderSky, if you want to roll the dice. You may want to get some quotes with shipping, see if you can get a break by having them dropped at a local motor freight depot, and take a good look at warrantees. While checking to see what the gremlins really did in your charging system.

Particularly, ask each mfr that interests you, how their gel and agm batteries compare for maximum life cycles. Sometimes those numbers contradict each other.
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Re: Gel battery weakness

Deka makes them in the US under license.
Sonnenschein Gel Cell Batteries
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Re: Gel battery weakness

The two current options are Deka and Trojan. The German curse word is what is in there now. I didn't think to look for them in the US.
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Re: Gel battery weakness

Gelled batteries, particulary in the GC-2 golf-cart configuration, are excellent batteries and a good choice for many boats.

The thing about voltage sensitivity is a bit overblown. Actually, a lot overblown. Assuming you don't stray far from the normal absorption and float voltages, they should last a long time.

I've got two gelled golf cart batteries in my basement which are approaching 15 years old and still test pretty good. Spent their first 10 years on a sailboat, the next two years in testing along with a bunch of other batteries, and have been just sitting around for auxiliary power and testing equipment in my shop.

BTW, the "charge divider" you mentioned is very likely a diode-based isolator. It drops the voltage about 0.2 to 0.7 VDC, depending on design. You can figure out how much by measuring input and output voltage levels while the alternator or battery charger is running.

Bill
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