12v question

[Seafarer_Bob]

I'm thinking about connecting my 36v trolling motor batteries to my 12v house for more capacity, anyone do this? The 36v / 60AH batteries are parallel so they'll add 360AH at 12v which should all but eliminate low voltage alarms when I'm not running. Right now the 36v batteries a're connected via a 50a breaker to "common" on the switch. "A" goes to the TM and thinking I'll connect B to a 36v to 12v /20a stepdown transformer, to a 20A inline fuse, to the positive house bus bar. Is this a bad idea or any concerns about grounding etc?

 

[Hookup1]

You don’t use a transformer to reduce DC voltage. Transformers require AC. You would have to use a power converter which is an expensive “fix” for low voltage alarms. Check engine charging and replace 12 volt house battery would be my recommendation (fixed mine).

If you do go the power converter route it should be fine for 12 volt house load. Grounds would be connected together. When you say the "batteries are parallel" which is what I would expect then you say "I'll connect B to a 36v to 12v /20a stepdown". With parallel batteries it wouldn't matter which battery positive you connect to.

If you are using heavy duty dredge reels like 12 volt LP's a power converter is not recommended.

 

[SkunkBoat]

Are you saying you have one 36V/60Ah battery or you have three 12vdc batteries in series to power your TM.?

If you have three 12v batteries powering your TM then you can, in some ways, rig it to also use them in parallel as 12v house.
One upside to using three 12v is simplifying using them as House.
The big upside is being able to charge them from motors.


You can't connect 36V to 12 V House. You will get a 36v House and break stuff. You can't step down using a transformer. Transformers work on AC not DC.

You would need a 12VDC regulated power supplyy that is fed by 36Vdc (38.4Vdc). I'm sure they make such a thing. Maybe that is what you are refering to when you say 36v to 12v step down. But its not a "transformer".

I just installed a TM and went with a single 36V vs three 12v. I can only charge from an AC powered 36vdc charger.
Not planning to look at using it for 12v house since I would not be able to charge House from motors. (again, there's probably a 12V to 36V charger out there...)

 

[Seafarer_Bob]

Are you saying you have one 36V/60Ah battery or you have three 12vdc batteries in series to power your TM.?

No, sorry if that wasn't clear. I have two 36v / 60AH batteries rigged parallel so that's 120AH dedicated to the trolling motor.
I'm saying use 36v to 12v converter they sell on amazon https://a.co/d/06nYCLEj and use that to connect the TM bank to house.
Just thinking I'll need a one way diod in front of house (2×12v AGM) to stop them from depleting the TM bank.

 

[SkunkBoat]

OK understand what you have now. The DC stepdown makes sense. Not so sure that paralleling the TM bank ( the 12v you get from the stepdown) with the 2-12v AGMs is a good idea.
You have a charger connected to house and the motors are charging. Too much to consider.
I'm assuming you have already separated 12v house from start batteries?

It would be a good backup house, using a switch. Not parallel, just switch one or the other, not both.
I can't imagine needing more than a pair of AGMs for House, unless you are running a fridge.
I run everything all night, on two lead acid grp24deep cycles.

I'm assuming the 36v batteries are LiFePo4?
You could ditch the AGMs entirely and use just the 12v you get from the 36v stepdown for house.
Save weight and expense of replacing 2 AGMs every 5 years.
That would reduce your TM run time though.

ahhh, Looking at that DC stepdown it is 20A at best. House is usually 40A

Again, charging is a consideration. Do you have a 12V alternator to 36V LiFePO charger to charge from motors?

I went with a single 36v/100Ah for my TM. have yet to get it wet...

 

[DennisG01]

We use "X"Voltage to 12V stepdown converters all the time when rigging electric outboards on pontoons (they run 24V or 48V). So, sure, that part is fine. Just don't get a cheapie one.

But... when the engine is running... and if I'm thinking of this correctly... you'll be back feeding 13V through the step-down (and to the lithium batteries?)... gotta think about that for a bit. It may not be an issue - just thinking outloud.

 

[Hookup1]

I wouldn't connect converter to the battery and charging shunt. You will have a voltage difference and current will flow. May actually draw battery down. Not necessary.

Put the house load on the common connection to a 1/OFF/2 battery switch. Connect the house battery with charging shunt from the engine to one position and the voltage converter +12 volt to the other position. This way you can select which source to use for house load and have a choice/backup plan.

 

[Seafarer_Bob]

Yes, the starting battery is isolated with an ACR switch. The 36v to 12v stepdown in the link above to amazon is 30amp, that's probably not enough to be my only house source, the windlass would probably cycle my electronics which is annoying.
My onboard charger is a Monster Marine "Ultimate Charger". Whether it's plugged into AC or charging off the alternator it charges the 12v house first and then when it senses they're full or starts charging the 36v system. The charger also has an aux button that in an emergency will deliver a charge from the 36v system to my starting battery for 30 minutes. That aux button must be a built in 36v to 12v step down, that's what gave me the idea to boost my house.

I know, probably a fire hazard but I think it's doable if I install breakers / fuses properly.

I just repowered from an old F225 to a new F250 which has a bigger alternator, maybe not as much of an

We use "X"Voltage to 12V stepdow13V through the step-down (and to the lithium batteries?)... gotta think about that for a bit. It may not be an issue - just thinking outloud.

... you'll be back feeding 13V through the step-down (and to the lithium batteries?)...

But... when the engine is running... and if I'm thinking of this correctly... you'll be back feeding 13V through the step-down (and to the lithium batteries?)... gotta think about that for a bit. It may not be an issue - just thinking outloud.

Thanks for the input, when the engine is running, the Monster Marine Ultimate Charger uses the 12v engine output to charge the 36v system, I'm not touching that.