Dumb question about deep cycle batteries in a 24v system

[hooah212002]

we recently upgraded our TM to a 24v system (65lb Minn Kota Maxxum). We did not buy 2 new batteries because the old one still worked, but I don't know how old it is. We took it out for the first time yesterday and it never lost power even though we used it to do a controlled downstream troll with the boat pointing upriver but traveling downriver slower than the current in a pretty fast river which required it to be at nearly full power to keep us steady.

 

When we got off the water, I put a multimeter on both batteries and the old one read something like 6.5v while the new one was still at 12.4. Both were fully charged at equal levels before we left. I've never measured that battery in years past so I have no idea if this always happened to it. I assume this means that the older batter is shot and should be replace, right?

[J Francho]

12.4 is discharged.  The older batt sounds shot.

[hooah212002]

2 minutes ago, J Francho said:

12.4 is discharged.  The older batt sounds shot.

That's what I was afraid of. I'm going to check it today to see if by some sort of miracle it is actually charging/charged because I really don't want to have to buy another battery before we can take it out again.

 

Danke.

[Chris at Tech]

On 3/9/2020 at 10:20 AM, hooah212002 said:

That's what I was afraid of. I'm going to check it today to see if by some sort of miracle it is actually charging/charged because I really don't want to have to buy another battery before we can take it out again.

 

Danke.

I know what you mean in saying this, but yes you really should want to buy another battery before you go out again.  If that voltage reading was correct, then you're likely looking at a battery that's damaged beyond repair.

 

Look at it this way -- if you *DON'T* get a new battery before you go out again, you're risking a day on the water without a working trolling motor.  Not sure what that would mean to your day of fishing, but it would ruin/shorten mine.

[papajoe222]

Using a volt meter isn't a good indicator of how well a battery is holding up.  You may get a satisfactory reading after charging, but the first time any load is drawn from it the voltage will drop dramatically and the 'good' battery will be overtaxed, not to mention the TM won't operate at maximum efficiency.

If the old battery has a dead cell or two, matters could get worse. You can pick up a deep cycle battery at any of the big discount stores (WalMart, Farm-Fleet) for under $100 and with proper care it'll last for up to five years under moderate use. 

[hooah212002]

On 3/10/2020 at 6:46 PM, Chris at Tech said:

I know what you mean in saying this, but yes you really should want to buy another battery before you go out again.  If that voltage reading was correct, then you're likely looking at a battery that's damaged beyond repair.

 

Look at it this way -- if you *DON'T* get a new battery before you go out again, you're risking a day on the water without a working trolling motor.  Not sure what that would mean to your day of fishing, but it would ruin/shorten mine.

It would certainly be a downer but we've been fishing for a while without a working TM so we're used to it. That said, it definitely shot up on my list of priorities. Getting out last weekend and having a reliable TM really changed how we fished and was really awesome. I did find myself getting greedy and pining for a fancy model with spotlock. A cable drive TM is a pain in the whozawhatzit when you don't have the foot pedal secured lol.

On 3/10/2020 at 7:42 PM, papajoe222 said:

Using a volt meter isn't a good indicator of how well a battery is holding up.  You may get a satisfactory reading after charging, but the first time any load is drawn from it the voltage will drop dramatically and the 'good' battery will be overtaxed, not to mention the TM won't operate at maximum efficiency.

If the old battery has a dead cell or two, matters could get worse. You can pick up a deep cycle battery at any of the big discount stores (WalMart, Farm-Fleet) for under $100 and with proper care it'll last for up to five years under moderate use. 

Without spending a ton of money, what is a better way to keep an eye on deep cycle battery health? Ideally, I'd like to have an onboard battery monitoring system with a phone app and the whole 9, but they seem to be rather pricey. Bear in mind that my dad already has plans on buying a brand new boat so this one is going to turn into a project boat in the next year or two so we really don't want to put a ton of money into it that doesn't equate to redoing the interior or things that can be reused (seats, TM/fishfinder, etc).

[Chris at Tech]

17 minutes ago, hooah212002 said:

Without spending a ton of money, what is a better way to keep an eye on deep cycle battery health?

Two suggestions...

 

First, bring the battery to an auto parts store or a place like a Batteries Plus.  Generally those places will test batteries for free.

 

Secondly, get in the habit of checking the water levels in your batteries a few times a year, and top them off if needed.

[WRB]

Always keep the 2 batteries in the same condition/age because batteries in series are only as strong as the weakest.

When you do this recharging time is equal and amp draw is equal.

You also need 2 circuit breakers, 1 40 to 50 amp breaker for each battery.

Tom

[Johnbt]

I'm curious why you recommend 2 circuit breakers when using 2 12v batteries for a 24v trolling motor. Minn Kota recommends using one circuit breaker and that's all I've ever used.

 

The drawing on this page references one circuit breaker in the drawing.

 

www.minnkotamotors.com/sites/minnkotamotors_site/files/selecting-the-correct-battery.pdf

[hooah212002]

On 3/13/2020 at 8:31 PM, WRB said:

Always keep the 2 batteries in the same condition/age because batteries in series are only as strong as the weakest.

When you do this recharging time is equal and amp draw is equal.

You also need 2 circuit breakers, 1 40 to 50 amp breaker for each battery.

Tom

I know one goes on the positive TM cable, but where would the 2nd fuse go?

[WRB]

47 minutes ago, hooah212002 said:

I know one goes on the positive TM cable, but where would the 2nd fuse go?

I have always added a circuit breaker on each positive battery cable to prevent a battery sort causing fire. Instead od a direct jumper from a positive battery terminate in series to the negative battery pole place a circuit breaker inline.

Cheap insurance.

Tom

[hooah212002]

36 minutes ago, WRB said:

I have always added a circuit breaker on each positive battery cable to prevent a battery sort causing fire. Instead od a direct jumper from a positive battery terminate in series to the negative battery pole place a circuit breaker inline.

Cheap insurance.

Tom

Ah, gotcha. I bought a 2 gauge jumper cable like this to connect the batteries: https://www.farmandfleet.com/products/g--549-deka-battery-to-battery-cable.html.

 

I looked, but did not see one that had a fuse.

[Johnbt]

I guess it wouldn't hurt, but I've never seen anyone do it.

[WRB]

You need 2 each jumpers with 5/16 terminal lugs to connect the resettable circuit breaker with 5/16-18 terminal studs when connecting 2 batteries in series. The circuit breaker to the trolling motor uses 3/16 terminal lugs for 6 gage wire. I also use a 50 amp switchable circuit breaker for the OB battery power and switch it off when charging the cranking battery.

Tom 

PS, you can get bye with 1 circuit breaker to the TM and hard wire the series jumper cable. The switchable and inline breakers are optional until you have a major problem like cooking the OB's computer or over heating a battery connection in the bilge with gasoline fumes.

[K1500]

You should always replace batteries in series as a matching set. Pairing your old bad battery with a new one is a good way to ruin your new battery.