Inverter on Bowthruster Batteries

[mariner62]

I'm considering the wisdom of hooking up an inverter, say 2kW, to my bowthruster batteries for use with my mains powered, occasional use, gizmos (like coffee machine, microwave etc). It seems to me that the bowthruster batteries get the occasional high power blast when maneuvering in marinas in a blow, but don't get used much at other times. Things like coffee machines and microwaves have similar duty cycles to a bowthruster - short term, high power usage - as opposed to longer duty cycle items like lighting, stereo, etc which run from my main service battery bank.

I'd probably look at putting a battery monitor on there as well so I can monitor the state of charge of the batteries, and make sure I'm topped up when heading into marinas etc.

Just wondered if anyone has any comment on the wisdom or otherwise of such a setup?

 

[pvb]

Most people's house banks are much larger in capacity than their bowthruster battery/ies, so it usually makes more sense to connect inverters to the largest bank. Not only is the depth of discharge lower for a given usage, but larger banks accept recharging more readily.

 

[greenalien]

Also depends on how fast you can recharge the bowthruster bank, and how heavy-duty the cabling is that connects it up to the alternator - using a microwave via an inverter will take the same amount of power as your bowthruster, if not more, and for considerably longer, so you'll probably have to run the engine to keep up with the discharge rate and will need cabling that can handle the necessary charge current.

 

[mariner62]

Most people's house banks are much larger in capacity than their bowthruster battery/ies, so it usually makes more sense to connect inverters to the largest bank. Not only is the depth of discharge lower for a given usage, but larger banks accept recharging more readily.

I agree, but its a close call on my boat as it stands. I've currently got 360Ah main bank and 240Ah on the Bowthruster - so not too much in it. If I don't end up connecting to the bowthruster bank, I'll probably end up connecting up at least another 120Ah battery to the main bank for extra capacity.

 

[Sandyman]

I cant see that your going to achieve very much.

 

[ianj99]

I agree, but its a close call on my boat as it stands. I've currently got 360Ah main bank and 240Ah on the Bowthruster - so not too much in it. If I don't end up connecting to the bowthruster bank, I'll probably end up connecting up at least another 120Ah battery to the main bank for extra capacity.

I'd stick with your original plan - at least there is no risk of discharging the main batteries.

 

[mariner62]

Also depends on how fast you can recharge the bowthruster bank, and how heavy-duty the cabling is that connects it up to the alternator - using a microwave via an inverter will take the same amount of power as your bowthruster, if not more, and for considerably longer, so you'll probably have to run the engine to keep up with the discharge rate and will need cabling that can handle the necessary charge current.

Actually that's a good point. I dont know what the difference currently is in terms of charging capability between the two. I'm hoping both are charging on the same alternator output, so it will be shared between them. That way, it shouldnt matter too much which bank is depleted, because it should get more charging current (all else being equal).

Would a bowthruster battery set normally be connected any differently for charging?

If the microwave is say 1kW, then thats about 90-100 amps from the battery bank. If I run it for say 10 minutes, that would be about 17Ah which doesn't seem too much for a 240Ah batter set. Add a cup of coffee or two at say another 25Ah per cup, we'd probably keep the charge level at over 70% of capacity. Seems ok to me as long as her indoors doesn't go nuts in the galley.

To put 70Ah energy back into the batteries may then take a couple of hours of charging at 35-40A. Hmm, that is quite a lot of engine noise for a coffee and tv dinner!

Maybe I should be looking at an upgraded Alternator too... let me see now ...

 

[mariner62]

Maybe I should be looking at an upgraded Alternator too...

Of course that would mean more charging current to the bows, and heavy cabling from engine to bows .. maybe not such a great idea.

 

[pvb]

If the microwave is say 1kW, then thats about 90-100 amps from the battery bank. If I run it for say 10 minutes, that would be about 17Ah which doesn't seem too much for a 240Ah batter set. Add a cup of coffee or two at say another 25Ah per cup, we'd probably keep the charge level at over 70% of capacity. Seems ok to me as long as her indoors doesn't go nuts in the galley.

To put 70Ah energy back into the batteries may then take a couple of hours of charging at 35-40A. Hmm, that is quite a lot of engine noise for a coffee and tv dinner!

Maybe I should be looking at an upgraded Alternator too... let me see now ...

Battery capacity is quoted usually at a 20-hour rate. So, theoretically, your 240Ah battery bank would deliver 12A for 20 hours. However, higher levels of discharge effectively reduce the capacity of the battery. Discharging at 100A will drop your effective battery capacity down from 240Ah to nearer 95Ah - less than half. So your microwaving and cups of coffee will bring the bank to less than 50% charged, which will adversely affect its life.

To recharge the batteries will also take much more than 2 hours, even with a larger alternator, as there's a limit to how much current a battery will accept, and the charging current reduces significantly as the battery charges up.

Your better bet would be to add another battery to your domestic bank, wire the inverter to that, and ideally run the engine when you use the inverter.

 

[mariner62]

Battery capacity is quoted usually at a 20-hour rate. So, theoretically, your 240Ah battery bank would deliver 12A for 20 hours. However, higher levels of discharge effectively reduce the capacity of the battery. Discharging at 100A will drop your effective battery capacity down from 240Ah to nearer 95Ah - less than half. So your microwaving and cups of coffee will bring the bank to less than 50% charged, which will adversely affect its life.

To recharge the batteries will also take much more than 2 hours, even with a larger alternator, as there's a limit to how much current a battery will accept, and the charging current reduces significantly as the battery charges up.

Your better bet would be to add another battery to your domestic bank, wire the inverter to that, and ideally run the engine when you use the inverter.

Copy all that, thanks. Looks like I will beef up the main battery bank, and maybe also look at upgrading the alternator/wiring to get a higher charging current. Much easier to upgrade a couple of meters of 50sqm than going to the bow. As you say, running the engine whilst using the high loads through the invertor is the sensible thing to do.

Thanks to all.