Dual Battery Voltmeter + Switch? (1 Viewer)

[jackhossross]

Hi everyone, working on my 100 Series.

Running a dual battery system that I want to monitor the health/charge of each battery from the cockpit.

I have a dual battery voltmeter set up, what I am concerned about is the voltmeter slowly draining the batteries. Should I be concerned about this?

I want to hook up a switch to turn the voltmeter on and off - for some reason I am having a hard time setting it up.

My voltmeter works that if the main battery isn’t receiving power, the aux doesn’t read. So I was thinking I could hook up a switch to that and control the on/off from there.

I can’t find the wiring diagram for this switch, but hoping to utilize the switch in the photo.

 

[e9999]

The voltmeter part itself would not be drawing much power at all, but the display lighting possibly could a bit more (although still minor I would think). But if you connect it to a circuit controlled by the ignition key, both sides should then be off with the key off based on what you wrote (if I understood you correctly) so then no switch needed and no issue at all? Or if both batteries grounds are connected together, just use the same ground wire for both sides and put a switch on that, could all be small gauge stuff?

 

[Mrtwooooo]

looks like you already have the gauges but the Blue sea mini OLED voltmeter is perfect for something to leave on all the time. The operating current is only 15mA (0.015amps) it would take 2.7 days to even use 1ah. You’d just need two of them to see both battery voltages.

Also maybe look at a Victron Energy Smart Shunt. Good way to measure the amount of current going into and out of the battery and it will also keep track of how many amp hours you’ve used.

 

[jackhossross]

looks like you already have the gauges but the Blue sea mini OLED voltmeter is perfect for something to leave on all the time. The operating current is only 15mA (0.015amps) it would take 2.7 days to even use 1ah. You’d just need two of them to see both battery voltages.

Also maybe look at a Victron Energy Smart Shunt. Good way to measure the amount of current going into and out of the battery and it will also keep track of how many amp hours you’ve used.

I grabbed the shunt - haven’t gotten around to installing it yet. Wanted to do their monitor but the whole kit is expensive.

 

[e9999]

IIANM, the smallest Victron Smart Shunt has a 500A capacity. FWIW, I calibrated mine, and -unsurprisingly given that rating- it showed a significant error (underestimation) below a few Amps. It's a great tool for solar applications, but may not be the best to keep track of the low currents we typically see in our trucks' house battery applications. Or.... just have fun with it!

 

[Mrtwooooo]

IIANM, the smallest Victron Smart Shunt has a 500A capacity. FWIW, I calibrated mine, and -unsurprisingly given that rating- it showed a significant error (underestimation) below a few Amps. It's a great tool for solar applications, but may not be the best to keep track of the low currents we typically see in our trucks' house battery applications. Or.... just have fun with it!

Calibrated it how? Or just compared it against a known amp draw?

 

[e9999]

Well, I didn't go to NIST, but I used several laboratory-grade ammeters.
As always, accuracy is a highly subjective concept, you decide what's good enough for your needs. I calibrated mine at fairly low currents, not because I thought I needed to do so badly in order to keep track of something like a fridge use for camping, but mostly out of curiosity and because I wanted to see if I could use it for other purposes. TBF, in practical terms, I would say it's likely fine to use it to keep an eye on your lithium house battery if you run a fridge, outside lights, stuff like that. Just don't expect it to be extremely accurate for draws under a couple of amps. In fact, from what I saw, when I went much under 1A mine stopped registering altogether. But, admittedly, who cares if you have a 100Ah lithium battery and a voltmeter. And if you run only lead batteries, then just a voltmeter is plenty good enough anyway, no real need for a BMS.

 

[sdnative]

IIANM, the smallest Victron Smart Shunt has a 500A capacity. FWIW, I calibrated mine, and -unsurprisingly given that rating- it showed a significant error (underestimation) below a few Amps. It's a great tool for solar applications, but may not be the best to keep track of the low currents we typically see in our trucks' house battery applications. Or.... just have fun with it!

My experience as well. The mA level draws for small parasitic devices like my BMS are enough to drain the battery enough over time (weeks) but not enough to for the shunt to track energy/capacity accurately. Pretty soon the actual battery capacity and what the shunt indicates are not in alignment.

If I am charging / discharging regularly with higher current loads (fridge/lights/etc), the energy/capacity measurements would probably work much better. My truck sits 90% of the time so the parasitic draws make the energy/capacity measurements not useful. I do use the shunt for charging/discharging current measurements though, which seems to work well.

I am using the smart shunt but the regular BMV-712 should be the same.

 

[spressomon]

I recently installed the Victron Smart Shunt 500A onto the LFP set-up in my little Escape 15A trailer and so far it seems to be tracking small draw from the 4 load balancers apart of the LFP. Although, I've "survived" the past 4+ years w/o the shunt and w/o drama just tracking voltage, so far I find it useful (if not completely accurate per above experiences).

 

[e9999]

Yes, there are a lot of interesting things to discuss about the Victron Smart Shunt but I should probably stop here. Maybe we should start a separate thread on those. Anyway, coming back to the OP, I would suggest that if you want to go BMS, you might look at less expensive and more current range appropriate devices, they are likely plenty good enough and maybe better for low currents. It's not like a shunt is rocket science. But personally, I would not do it because of a real need, I might but for me it'd be more for fun. Couple of voltmeters are a good start. And a little switch is easy to put in, likely nothing compared to opening the consoles up. And if you want to make things simple for you, have a look at wireless remote switches. Should be very inexpensive for no current to speak of.

 

[jackhossross]

Yes, there are a lot of interesting things to discuss about the Victron Smart Shunt but I should probably stop here. Maybe we should start a separate thread on those. Anyway, coming back to the OP, I would suggest that if you want to go BMS, you might look at less expensive and more current range appropriate devices, they are likely plenty good enough and maybe better for low currents. It's not like a shunt is rocket science. But personally, I would not do it because of a real need, I might but for me it'd be more for fun. Couple of voltmeters are a good start. And a little switch is easy to put in, likely nothing compared to opening the consoles up. And if you want to make things simple for you, have a look at wireless remote switches. Should be very inexpensive for no current to speak of.

For whatever reason, I can't get my switch to work. I've managed to wire up my full 12v system, but this darn switch doesn't want to turn on and off the voltmeter. Quite annoying haha.

 

[e9999]

I imagine you tested continuity on the switch poles?

 

[TomH]

A few years back @george_tlc dissected one of those voltage gauges and determined that removing a diode killed the power to the display. I copied what he did and it's been working fine. Not sure if your gauge is the same but might work.

MICTUNING double voltmeter

anyone used one of these yet? will it fit the knockouts of an 80 series? are they junk, should I pass? https://www.amazon.com/gp/product/B072QZPKLG/ref=ox_sc_act_title_1?smid=A15H3UKRB6RHF9&psc=1

forum.ih8mud.com

 

[PeterTheBeater]

I have the same voltmeter in my car. The way it works, is that there's a "primary" voltage and a "Secondary" voltage. The display and circuit gets it power from the "primary" voltage. So even if the secondary battery is hooked up, it wont turn on.

The way I have mine hooked up is I have it wired in into an ACC relay. So it's off when the power is off, and both displays come on when the ACC+ is on.

Also, this is such a small draw it wont really make a difference. Your clock draws the same power.

 

[jackhossross]

I have the same voltmeter in my car. The way it works, is that there's a "primary" voltage and a "Secondary" voltage. The display and circuit gets it power from the "primary" voltage. So even if the secondary battery is hooked up, it wont turn on.

The way I have mine hooked up is I have it wired in into an ACC relay. So it's off when the power is off, and both displays come on when the ACC+ is on.

Also, this is such a small draw it wont really make a difference. Your clock draws the same power.

So if I connect the "main" to the cigarette lighter positive wire, it will read an accurate current and only turn on when the car is turned on?

 

[PeterTheBeater]

So if I connect the "main" to the cigarette lighter positive wire, it will read an accurate current and only turn on when the car is turned on?

It might not be 100% accurate, but it will be good enough. I'm referring to switch that you have in your console.
My "primary" is tapped off ACC wire, and "secondary" is tapped off my ACR. That's how it works for me.

 

[PeterTheBeater]

I'll see if I can pop mine out later today and verify that's how I wired it up.