battery exchange

[BobFino]

Hello, my boat has two 8D batteries for my bow thruster. I realize it is a 24 volt system but I cannot find a definitive answer on why I cant use two group 27 or 31 instead of 8D's. The batteries are only for the thrusters. Any thoughts on this subject would be appreciated.

 

[bobeast]

It's all about Amp/Hours. A typical 8D battery has between 200 - 300 Amp hours of capacity. The smaller batteries, between 75 and 110 Amp Hours. To get the equivalent capacity using 27 or 31, you'd probably need to double up. That is to say two parallel batteries in series with another two parallel batteries, for a total of 4 smaller batteries to replace the two larger ones. Of course this leads to additional wiring connections, more maintenance, and probably will take up more space than your original 8D batteries.

All that said, there is no electrical reason you couldn't do this. Your thruster doesn't care where its power comes from, only that it has enough.

Now there are some high capacity Lithium batteries in the smaller size that could be used, but that right there is a whole 'nuther can-o-worms.

 

[bbwhitejr]

On our 44DB, had 2-Grp 31s on the thruster and never an issue.

Bennett

 

[dtfeld]

The likely reason the 8Ds were specified is likely related to the amount of current the batteries are expected to deliver to the thruster in relation to the size of the battery bank. This is the C rate or the charge/discharge rate. Most times your trying to keep the C rate around a max of 1. C rates greater than 1 will degrade the life of the batteries.

You can calculate the C rate on you thruster batteries if you know the amperage the thruster draws and the Amp hours of your bank.
C=Amp draw thruster/Ah battery bank.

If looking to go with a smaller bank, your C rate would increase, and the longevity decrease. But if you looking to go smaller due to the weight of the 8D batteries, using a higher quality battery would help. Or you can solve this by using 4 smaller G31s.

 

[km1125]

Unless you're leaning on that thruster switch a lot, a pair of 8D's is really overkill. You might need a couple hundred amps for the thruster, but you're typically only using it seconds at a time. You typically need "cranking amps" from the thruster battery bank rather than "amp-hours"

In the "worst case" conditions, what is the longest you'd EVER run the thruster? 10 times for 10 seconds each? More?
Just as an example, running 10 times for 10 seconds on a docking maneuver with a thruster requiring 300 amps would only result in about 8.3AH of capacity.

 

[BobFino]

Really appreciate all the info. I typically do not use the thruster much at all. The bow thruster requires 24 volts and 540 amps. The group 27 battery has 90amp hours and 705 MCA. So, I am thinking two group 27 deep cycle battery would be sufficient for what I need. Is my thinking correct?

 

[bbwhitejr]

Yes it is.

Bennett

 

[Tim Fila]

Those 2 batteries are probably less weight than one 8D.

 

[ranger58sb]

Hello, my boat has two 8D batteries for my bow thruster. I realize it is a 24 volt system but I cannot find a definitive answer on why I cant use two group 27 or 31 instead of 8D's. The batteries are only for the thrusters. Any thoughts on this subject would be appreciated.

We repurposed our thruster batteries by replacing the charger with an inverter/charger.... so now we can run fridges/freezer, outlets, and small appliances (microwave, coffeemaker, etc.) without having to start the generator.

Which in turn means we only need to run the generator for AC, the water heater, and probably twice a day battery charging (coinciding with meals) when at anchor.

-Chris

 

[BobFino]

Again, thank you for the clarification. Yes, the two group 27 are the weight of one 8D. Boat might have a list lol

 

[LMBoat]

Distance from the batts to the thruster? That could be the reason for the 8D's. I'm guessing the 8D's are in the engine room with a long wiring run to the thruster. You could use smaller AGMs (or some type of closed batt) close to the thruster with shorter/thinner wiring runs. Ck to be sure, just my thoughts.

 

[km1125]

Distance from the batts to the thruster? That could be the reason for the 8D's. I'm guessing the 8D's are in the engine room with a long wiring run to the thruster. You could use smaller AGMs (or some type of closed batt) close to the thruster with shorter/thinner wiring runs. Ck to be sure, just my thoughts.

That'd be a great reason to go to a 24V system, but doesn't really matter to the "size of battery" issue. It could be a decision between using a 12v battery really close to the thruster or a 24v battery that needs to be placed far away (like in an engine room).

 

[BobFino]

Hmm, it is a 24-volt system and yes, the batteries are located in the engine room. According to MaxPower the manufacture of the bow thruster, it requires a minimum of 22 volts to work correctly. Judging by the gauge of the wire they used, I do not believe I will have a current issue.