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Dual battery help


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Hi guys,
I've been having problems with my dual batteries.... well one of them anyway I think. I have a battery selector that came with the boat, which has 1, 2 both and off. When I start my boat I put the selector onto both, leave it on this switch while I'm scooting to wherever I go, when I stop and I run the fishfinder I switch the switch it to battery 1. Yesterday at different times for some reason there wasn't even enough power in battery 1 to run the fish finder on its own, but at different times it did run and it ran for a while.

Should add that when I select both on the battery selector to start the outboard I have had no problems starting the outboard at all.

I'm not really an electronics guy and I am completely new to boating and especially dual battery setups and how they work - or how they best work. But I decided to pull the batteries out and have a look at the make etc so that I could describe them in the hope it would be easier for one of my fellow fishraider forumnites to maybe tell me what they think the problem may be.

So below is a description of the batteries and they seem to be wired to each other and the battery selector.

One of the batteries (which I'm pretty sure is battery 1) is a motogaurd battery which says on the label that it has 270 CCA RC 50 and nothing else I can see like how many AMPS etc. It's negative lead is connect straight to the other battery's negative terminal. It's positive lead goes into the battery selector.

The other battery (which I believe is battery No 2) is a sea master says it has 580 CCA 130 minutes RC MAX recharge 14.8. I believe this battery might be from either 2004 - or 2005 as there is a plate on it which says 12 months warranty from the date of purchase which has a list of months from Jan through to Dec and the years 2003 - 2004 - 2005 as the years listed.

I tested the batteries with a multimeter this morning to see what they read.
Battery 1 (motoguard) had a reading of 12.4 volts.

Battery 2 (sea master) had a reading of 12.7 volts.

I should add that the connections looked a bit how ya going also, so I cleaned them up with some 600 grit sandpaper to give them a better connection.

Any advice as to what the problem might be would be heaps appreciated.

Edited by garfield28
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Guest Aussie007

it could be those "how ya going" terminals mate often they are a simple problem and cleaning them doesnt work u just need to replace them im no electrician and dont like electronics but i think on a duel battery set up u must first charge your batteries for the day than use 1 battery to start the engine and maintain electronics run that battery down and use the second to start your engine later on so u dont end up stranded, by running around with the switch selected to charge two batteries at once i think your halving the amperage to charge both instead of one battery at a time

ive never had a duel battery set up but thats the info ive takin on board about a duel battery set up

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Hi Geoff,

1)First thing to do is give them a charge while disconected and then take them to an auto shop who can test them with load.

What size outboard you running?

sounds like you are set up with a smaller battery to run electronics when stopped and a larger one for starting.

If you go outside then i would be using the same size larger battery for the backup.

I use 2 100AH batteries (marine) and an auto switch relay /isolator (about $190). Select the battery you wat to run off and it charges both but only runs off the one you select . If it goes flat or fails then you just switch to the second battery and your away.

You don't have to use this but it saves you forgetting to swich between batteries all the time.

In saying this it could just be wiring is the problem. In which case you just need to check all conections.( can take a while)

Hope this is of some help.


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Hey mate I'm pretty sure batteries in this set up should be the same rating. It's kind of like a strong arm and a weak arm, the bigger arm will carry the weaker and there not balanced.

To test the batteries properly they need to be under load. Maybe disconnect the fuel line and crank the motor over while having the meter on each battery and see what voltage your getting there.

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I don't know a great deal about dual batteries but have exactly the same set up as you.

For me I select one battery per outing and leave the other for emergencies. ( like a spare can of petrol) if I need to use it it's time to head home. As I have no pull start. I alternate batteries each outing and give both an overnight charge between outings.

I've never had a battery run out of charge on an outing for me and leave all my fish finder gps and radio on all the time.

......... SaltWaterDog

Edited by NaClH2OK9
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Guest Aussie007

forgot to add i think when a battery is rated in cold cranking amps (CCA) its a car battery? marine batterys seem to be rated in amp hours (AH) so u might just have a couple duc car batteries in your boat and 270CCA is a tiny battery for a little gogo moble or something lol

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battery number one sounds tiny (cca 270 cold crank amp) and is probably a $40 car one.

Really, they don't last and in a boat won't last very long at all.

Regardless of whether it is any good or not, I'd replace it with something similer to

the other one. 4WD is better than car, and Marine is probably better again.

I've not really come to a good understanding about the Gel/AGM/slow discharge batterys

yet, other than normal batteries get used to start, then expect to be recharged staright

away and with not much use on them during the day.

The others (deep cycle) are designed to get flat through the day then be recharged when you

get home.

I also think it is a good idea to have a portable jump starter battery that you can take when you go out.

As far as I know, you wont be able to push start your outboard in the water ;-)

THe other advice is sound. Charge the batteries before you leave is good.

my 2cents.

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Thanks for all the relies guys.

I guess the general conciseness is I should have both batteries the same size and with the same amount of CCA's?

I read somewhere else this morning that I should never start the motor with the switch selected to both. It should only be in emergencies?? Do you think by starting the boat on both that I have been drawing all of the juice out of my smaller battery and then when I switch to it to run the sounder and gear it has had most of its power drawn out of it?



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I think some would say that starting with two batteries means that you can flatten

them both. I personally cant see anything wrong with it. I'm not a mechanic and not

an electrician so take that with a bit of salt.

I'd say that using two battery's will mean that you can try for a bit longer to get it going

and lets face it, you've probably already started the outboard the night before you

decided to go out anyway so you'll have an idea about whether you are going to have issues or not.

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