Charging trolling batteries from engine alternator

[ajax5240]

I am setting up a 24v trolling motor system. What would I need in order to have the trolling batteries charge from the alternator when the engine is running?

I'd prefer to do it without breaking the bank too much.

Thanks for the help!!

 

[Silvertip]

Re: Charging trolling batteries from engine alternator

You are (were) a Subaru Master Tech you should have the answer for this. But since you asked -- the answer is a qualified NO! Think about it. You have a 12 volt alternator and a 24 volt trolling system. Simple you say, just connect a pair of hot and ground from the alternator to each of the two batteries. While that may sound logical, draw a diagram and you will see that that jumper wire that creates the series connection for the 24 system also creates a dead short across one battery when you connect the alternator. You could add a "really heavy duty" DPDT switch that connects the hot and ground from the alternator to each battery INDIVIDUALLY but you cannot charge both batteries SIMULTANEOUSLY. You must switch grounds to avoid the shorting situation.

Lastly, what is the charging system output of this engine. Keeping in mind that a 24 volt troller draws 40+ amps/hour at its fastest setting, if the alternator has a low output you would need to run for hours to make up that used current. We need more details on your system but basically, without an extensive switching system, you cannot charge a 24 volt battery bank from a 12 volt source.

 

[ondarvr]

Re: Charging trolling batteries from engine alternator

There are chargers from Minn kota and others that handle the 12V to 24V issue for you, I think they run about $90.00 or so.

But like already said, at best you will just slightly lengthen the time you can use the trolling motor unless you run for several hours to get where you fish.

 

[ajax5240]

Re: Charging trolling batteries from engine alternator

Yea, I know that I can't just hook the alternator lead to the 24v system batteries.

I thought there were charge control boxes that could handle the task, but haven't found one with some extensive google'ing.

We fish day trips about 95% of the time, so I don't run out of power really ever. There are some times that my current 12v with a 55# MK will run low and lose top speed. I am hoping that i will have longer life on two group 29s on the 24v 80#.

 

[ondarvr]

Re: Charging trolling batteries from engine alternator

MK 2 DC Alternator Charger (2 bank)

I have a 24V 80lb Terrova and can run it for 7 hours straight and still have plently of power left.

 

[Todd4]

Re: Charging trolling batteries from engine alternator

I have the MK 3-bank DC charger. First it will completely charge my start battery, then switch over to charge my 3 other batteries (house and 2 TM batteries). It keeps all batteries isolated from each other so any charged battery will not be sucked down by any discharged battery. However, it's unlikely that your alternator can keep TM batteries charged up, unless you run for hours returning from trolling. But every little bit helps. I still use a AC 4-bank battery charger when back at shore to keep all batteries fully charged and topped off.

Todd

 

[ajax5240]

Re: Charging trolling batteries from engine alternator

Yea, after looking at the prices of the on board alternator chargers I don't think the extra little bit of battery juice we would get is worth that cash. Our longest reservoir is about 5 miles long. Even a run from one end to the other wouldn't do much. Only time it may get a bit of a charge is when trolling with the gas.

For the $130 I could buy a backup battery, but just one won't really help much on a 24v system. And I'm not carrying two extras.

We would probably be out of beer before the two batteries run too low anyway.

 

[fishrdan]

Re: Charging trolling batteries from engine alternator

Our longest reservoir is about 5 miles long.

With only 5 miles of running the main you won't get much charge from an alternator, unless you troll off the main for a long time.

MK's DC alternator chargers have a low 10 amp/bank output, so it's not a real solution unless you run looooong distances. Even at that, it won't top off the trolling batteries, only put some run time back in them.

I fish big reservoirs and have my trolling motor batteries setup to charge off the main. IE: charge the 24V bank while trolling off the main. The trolling motor batteries get their series connection through the trolling motor plug, so with the TM plug removed, they are 2 separate 12V batteries. Inserting a 12V charging plug (paralleled) into the socket allows both batteries to be charged at the same time,,, since one of the batteries is connected to the start battery through an A-B battery switch. It's a bit cumbersome swapping plugs and flipping switches, but works.

With the scenario of only running the main for 5 miles, the only realistic solution would be bigger or more batteries. It really depends on the size and weight of your boat, and the battery capacity. I use a 24V 70# MK on my 14' jon boat and have never run out of power, even when fishing multiple days. I use the same 70# TM on my 18' Deep V and only get 6 hours before the battery power is dropping off. One of the guys down here trolls a glass 21'er all day off an electric TM, he has 4- group 31 batteries in a series/parallel configuration.

 

[ajax5240]

Re: Charging trolling batteries from engine alternator

That is some serious battery power! 4 group 31's Holy COW!

I run a 55# Powedrive on one group 29 and get all day out of it, we bass fish so the trolling motor just runs in "blips" to correct our course and run along the banks at slow speeds. I figure with the upgrade to a 80# and two group 29's I should be ok. If there was a good option to re-fuel them it would be worth looking into, but I think I'd rather put that cash into an upgrade to Optimas or a group 31 from the 29's.

Thanks for all the help and advice!