BM1 battery monitor and 2 batteries, with 121 switch, best way to set up.

[steve yates]

I know that a bm1 cannot properly monitor 2 separate batteries, but could monitor a battery bank of 2 batteries. From reading posts on here, it seems that would only work if the battery switch was used on both all the time ? as switching from 1 to 2 will bugger up its calculations.

i have also read that it can actually monitor one battery, and give the voltage of the second battery?

They are both 115ahr leisure batteries, no engine start batteries here as its an 18ft boat with a simple outboard for engine.
one battery is about 5 years old, tho hardly used for 4, the other is brand new. They will be fed with a 50w solar panel to recharge when back on the mooring.

My idea is to use one of the batteries as the main one, and when the monitor says its getting low in a/hrs, switch to the other battery. I can live with just voltage on this one to let me know when its time to find a marina and plug in a battery charger.

I do want to monitor amps in and amps out when instruments are in use.

What would be the best way to set this up?

Thanks.

 

[PaulRainbow]

Best bet is to forget the 1-2-b switch and wire the two batteries in parallel Steve. They'll charge faster and get an all round easier life. Simpler and the BM1 can monitor them without fiddling around with switches.

 

[VicS]

I know that a bm1 cannot properly monitor 2 separate batteries, but could monitor a battery bank of 2 batteries. From reading posts on here, it seems that would only work if the battery switch was used on both all the time ? as switching from 1 to 2 will bugger up its calculations.

What would be the best way to set this up?

Thanks.

If you insist on separating them with a 1, 2, both switch or even separate switches, and want to monitor current in and out to each separately you will need two monitors, but you could use 2 BM1 Compact monitors as each will monitor the volts on its associated battery.

Otherwise wire them to a standard BM1 in the same ways as the installation instructions show for a services and an engine start battery. It will give you voltage readings for the second battery but not current readings for it .

If you connect them in parallel, scrapping the selector switch, a BM1 Compact monitor will be adequate but you will have current and voltage readings for the combined bank

 

[steve yates]

Best bet is to forget the 1-2-b switch and wire the two batteries in parallel Steve. They'll charge faster and get an all round easier life. Simpler and the BM1 can monitor them without fiddling around with switches.

Its already setup paul, and I do want to select the batteries individually, so if i leave it on by accident I only drain one and not both
Vic, yes its a compact one that I have, thats a thought, I might keep an eye open for a second one going cheap, but in the meantime just monitor the one battery and the voltage on the other. Its still far more than there ever was on this boat before, ie nothing at all.
I also have one of those little round usb style. outlets with a voltmeter on it kicking around, I could just use that on the second battery And just wire the bm1 up for the first one.

 

[Graham376]

Its already setup paul, and I do want to select the batteries individually, so if i leave it on by accident I only drain one and not both

If you connect all the boat loads to the load output on the solar regulator, it will cut power off when batteries drop to a predetermined level so you can't totally discharge and knacker them.

 

[Ningaloo]

In my experience, most (older) boats which have retained a battery selector are often using two similar sized batteries, often 100Ah each or less. This means that only about 30-50Ah capacity is available before starting to damage the battery. Probably not sufficient to run an autopilot let alone a fridge on a day long passage.
Ideal solution is to combine your two batteries into a dedicated domestic bank which will double the capacity available.
Get a small (motorcycle?) battery to start the engine. It needs minimum capacity so can be small enough to fit in the engine bay if there is no room elsewhere. Just needs to have sufficient CCA to start the engine. The starter battery never gets discharged as minimal amps are used to start the engine and once that job is completed the alternator immediately recharges it.
You just need a VSR or similar to isolate the two battery banks. Not difficult or expensive to do and in my opinion a better investment than the BM1.

 

[Daverw]

Ningaloo, he has an outboard so no starter battery requirement

 

[peteK]

If you connect all the boat loads to the load output on the solar regulator, it will cut power off when batteries drop to a predetermined level so you can't totally discharge and knacker them.

Would it work at night

 

[[70521]]

Run a positive lead up from each battery to a convenient point and terminate neatly. Take the positive lead from the BM1 and move between the positives from the batteries. This assumes you have a common negative.

 

[Tranona]

Not sure why you are reluctant to throw away the switch as it serves no useful purpose. How are you going to leave a switch on accidentally with enough loads on it to drain a battery? Just one isolator that you automatically turn off when you leave the boat. As already pointed out using just one battery at a time seriously limits your usable capacity, although I can't see what you might have in an 18' boat that will deplete a 200AH capacity bank. You don't need an "emergency" battery for engine start, so just keep it simple.

 

[steve yates]

I'm fine with the switch, I just happened to have one in the garage so used it. its just a glorified isolator, so dont feel the need to change it out or spend cash that can be used for something I dont have.
The set up is simple and I have no engine to worry about starting, but 5 days sailing at anchor with autopilot used, diesel heater running at night, charging phone and using ipad as plotter so permamently plugged into a usb outlet will soon make a fair dent on my battery capacity. I've tried set it up to maximimse the days I can go without requiring a marina power hook up to plug in a battery charger.
Thanks all, I think its clarified for me, i'll use the bm1 on 1 battery and get another at some point for the second battery.

 

[Graham376]

Would it work at night

I don't use those connections and long time since I read the instructions but, from memory, the power cut off is governed by battery voltage dropping and nothing to do with solar charge (or not) at the time.

 

[Graham376]

Steve, With a 1,2, both switch and wanting to keep the batteries separate, how are you going to charge them both from solar with only one regulator, without paralleling?

 

[[70521]]

Steve, With a 1,2, both switch and wanting to keep the batteries separate, how are you going to charge them both from solar with only one regulator, without paralleling?

Perhaps another 1-2-1 switch?

 

[steve yates]

Steve, With a 1,2, both switch and wanting to keep the batteries separate, how are you going to charge them both from solar with only one regulator, without paralleling?

I have a dual battery controller, wont that do the job? I thought the solar controller gets wired directly to the batteries, and the controller is responsible for cutting off charge when necessary. Am I wrong?

 

[VicS]

I don't use those connections and long time since I read the instructions but, from memory, the power cut off is governed by battery voltage dropping and nothing to do with solar charge (or not) at the time.

It will depend on he controller. What you descibe is common but there are controllers than can be programmed to do different things with the " load " output

Steve, With a 1,2, both switch and wanting to keep the batteries separate, how are you going to charge them both from solar with only one regulator, without paralleling?

I have a dual battery controller, wont that do the job? I thought the solar controller gets wired directly to the batteries, and the controller is responsible for cutting off charge when necessary. Am I wrong?

With a single output controller it will be possible to charge either one battery or the other or charge both in parallel. The downside will be having to leave the selector switch in an "ON" position to charge the batteries.

A dual output controller can be connected directly to the two batteries and it will charge them even when the selector switch is "OFF" ITYWF it will be a PWM controller rather than a MPPT one

 

[Graham376]

I have a dual battery controller, wont that do the job? I thought the solar controller gets wired directly to the batteries, and the controller is responsible for cutting off charge when necessary. Am I wrong?

I've never used a dual battery one (I used two separate ones) but think what you say is correct, others will be along quick enough if we're wrong

 

[Tranona]

I'm fine with the switch, I just happened to have one in the garage so used it. its just a glorified isolator, so dont feel the need to change it out or spend cash that can be used for something I dont have.
The set up is simple and I have no engine to worry about starting, but 5 days sailing at anchor with autopilot used, diesel heater running at night, charging phone and using ipad as plotter so permamently plugged into a usb outlet will soon make a fair dent on my battery capacity. I've tried set it up to maximimse the days I can go without requiring a marina power hook up to plug in a battery charger.
Thanks all, I think its clarified for me, i'll use the bm1 on 1 battery and get another at some point for the second battery.

If it helps, I have 3 isolators (one new still in its packing) and a 1/2/B switch redundant which you are welcome to have for nothing as they are going in the skip anyway. Do you have 2 outlet mains battery charger? - expensive things, but with one bank you can have a one outlet charger which will save you about 10 times the cost of a single isolator.

Just saying.

 

[Ammonite]

I'm pretty sure the BM1 has an alarm function if you are worried about flattening the batteries. Personally I'd parallel the batteries and keep it simple as others have said. Seperate batteries on an 18ft boat with no inboard doesn't make sense IMO

 

[steve yates]

A dual output controller can be connected directly to the two batteries and it will charge them even when the selector switch is "OFF" ITYWF it will be a PWM controller rather than a MPPT one

Yup, thats what I ordered.

Im not quite getting this I think.
Precisely what is the advantage of paralleling the batteries as opposed to just selecting b1 or 2? Are there tangible advantadges to doing that in terms of using my battery monitor? And are there any disadvantages?
Bearing in mind I already have a dual battery solar controller too.
I have no problems with a 121 switch by the way, both my big boat and my mates boat have them and I am well used to them, so convenience in turning on and off is not a good enough reason alone