Electrical Systems/Battery SG

centerline · Oct 4, 2014

jrevkin said:
I'm surprised there is no electrical systems forum, so I'll post here.
I have 2 battery banks. One starter battery (Battery 1) and Two house batteries (Battery 2 and 3). My battery selector has Battery 1 (1), both (1,2,3) and Battery 2 (2 and 3 in parallel).

They are all Interstate SRM 27/12 Volt Deep Cycle Marine Batteries
Cranking amps 750
cold cranking amps 600

I bought the boat about 2 years ago and the prior owner indicated that the batteries shold be about about 3 years old to this date.

I keep them charged at the dock, connected to 120 v AC.

The last few times I've gone boating, after shutting off the engine, with the batteries set to house, I've lost electrical power after only about 30 min. The voltage is fine when the engine is running, for all 3 batteries.

I checked the specific gravity today and the two house batteries (Battery 2) seem to have a lower SG than the starter batteries.

Here are the SGs corrected for temperature for the 18 cells

1 2 3
1252 1266 1266
1307 1267 1267
1292 1267 1269
1297 1268 1218
1282 1267 1142
1297 1267 1267

It would appear that I have one bad and two marginal cells in Battery 3.

Questions.

Why would the 2 house batteries have lower capacity than the starter?
Is it unusual to have batteries like this, lose their life after 3 years? (I disconnected the batteries over the winter, but did not have them on trickle charge)

Is it time to replace all 3 and if so, I'd be interested in pros and cons. I have a battery box that just fits these 3, each measuring 12.75 X 6.75 inches.

re: relays and monitors

I have a macromatic voltage monitor relay and Xantrex Link 20

Thanks so much.

Jim
Branford, CT

Shannon 28, SV SPIRIT
Starboat, Folli 8229
Finn Dinghy, Devoti USA 911
Laser, USA 191454

You write "one bad and two marginal cells in battery three".... i am assumingbattery three is a house battery so its definitly going to pull down battery two no matter what position the battery switch is in, but when the switch is in the combine position it will also pull down the starter battery....

Some people tend to beleive that two good batteries will carry the "bad" third battery, but infact the bad third battery will bring the other two down to its level.

The system seems to have no problems when the engine is running because you have a good alternator to supply your needs while its working. BUT... its being over taxed trying to push juice into a bad battery. Its harmful to the system and is only a matter of time til the charging system gives up...

You must replace the bad battery.

Any Battery can suddenly fail at any time, so if your other batteries are good, but three years old, the options would be to replace the them all, OR... only replace the two that are together, OR... replace the bad one with a good "used" one and try to get a couple more years from them...
But i would never replace one battery in athree year old bank with a new one unless there was no other options available... because of the expense of the new one and it isnt going to boost the system, but will be drawn down to the age level of the batteries next to it.
Also to keep in mind when deciding how best to replace the batteries is, how much damage may have been done over how long of period to the the "good" batteries in an attempt to carry the bad battery.

A 30 minute draw down may seem fast and unreasonable for three batteries, but it actually isnt when one of them has internal damage.

centerline · Oct 4, 2014

jrevkin said:
Why would the 2 house batteries have lower capacity than the starter?
Is it unusual to have batteries like this, lose their life after 3 years?

Unusual?... not at all when you have a bad cell in one of the batteries.

"one bad apple will spoil the whole barrel"...
This old saying holds true for more than just fruit.

The 2 "banks" have been disconnected from one another by the switch... so its possible and common for the house bank to be be drawn down beyond its useable recommended draw limit of 50% and this will take life from the batteries.... also, in your case of the batteries 2-3 bank, the one battery is connected parallel to a bad battery.... the bad one is going to suck the life out of the good battery.....

Stu Jackson · Oct 4, 2014

Bite the bullet and get two new house bank batteries. Even you don't know how the PO treated his batteries.

Benny17441 · Oct 4, 2014

A bad battery will drain any batteries connected to it. Think of it as if electricity flows downhill like water and you may visualize the power flowing from a full battery to an empty one. I would eliminate the bad battery and move the remaining good battery to the starter position and pair your current starter battery with a new one of the same capacity for house functions. The reasoning behind it is the starter battery does not have as many charge/deep discharge cycles and will pair better with a new one. The one formerly paired may have suffered some abuse and will have a limited remaining useful life but is likely still good for the easier task of starting the engine. I agree batteries should last more than three years but we cannot rule out that manufacturing defects will have some fail prematurely. You could replace the whole house bank but I do not see the added expense.

Electrical Systems/Battery SG

jrevkin

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