dual battery level monitor

Hey guys,

I would love some guidance as to how to implement a battery level monitor for two batteries. I’m not looking for the voltage but rather what percentage of the battery is left. This will be monitoring two deep cycle batteries on a boat. when I’m not wakeboarding we are often just listening to radio with the engine off. I would love to have a monitor that would tell me when the batteries are getting low and also (possibly) switch from one battery to the other at a certain point (as to not go into the deep cycling in order to preserve the batteries health). i’ve found a ton about displaying how much voltage; but In all my searches, i haven’t found much about the battery level.

thanks in advance,

scott

You probably won't find much about it, because for most battery types there's no easy way to measure how much charge there is in the battery. You can monitor the voltage and see that decline when the battery finally goes flat (which is probably all you need for your application) but if you want to know how much charge is in a lead-acid battery the only solution I can think of is to monitor the current in + out, integrate that to calculate the charge added/removed, and subtract that from an assumed capacity and initial charge state. Of course, if the battery capacity is less than you expect or it didn't start off fully charged, the answer will be garbage.

You can't determine battery level with voltage alone unless you let them rest for hours with no load.

Normally you use a technique called "Coulomb counting", in a nutshell this means counting the amps in and subtracting the amps out plus a fudge factor which for wet cells is about 1.2.

Eg you use 1A you have to put 1.2A back in.

Once you know the state of the battery you can switch it over using a relay (small one of you are only using a radio, BIG one if you are starting from either battery.

All that said this is the same issue 4x4 people face, and to my knowledge nobody does this. You start the motor from one battery and use a second one for "house" stuff. The two are disconnected when the ignition is off so you can never flatted the starter battey. You can just use a switch, a relay driven by the ignition or there are smart gadgets that test level and only disconnect/connect when the time is right.


Rob

thanks guys. im thinking things will get pretty muddled up when im charging from the alt and running the batteries. maybe more effort then it's worth. I do have a switch that lets me isolate each battery, combine them or disconnect both of them. maybe i'll modify my goal.

just switching to battery 'A' when cranking, battery 'B' when the engine is off, battery 'A' and 'B' while the alternator is charging and the. all off when its time to shut down.

You mention a Relay, i wasn't aware that a relay could handle that amount of current. i think my suv uses some sort of solenoid to switch on/off it's cranking battery. The relay sounds cheaper. I'll have to research

found some and they are much cheaper. thanks :slight_smile:

My guess is those relays need to be “heavy duty”. Lead acid battery can produce a lots of currents ( 30 A to 100 A ) when they start-up an engine… I assume a boat engine a way bigger than a car, mostly like a tractor trailer truck… in my opinion…

edit : those will do…200 A but at $60.00 each <— :astonished:

the engine is an indmar (aka chevy) v8. i agree that it needs to be heavy duty. I'm hoping that the 200 amp versions would have plenty of headroom while cranking.

I agree with you.

engine is an indmar (aka chevy) v8.

Humm, muscle car engine... Cool !!

But the $60 price tag ??? Man....

i think I could get away with wiring 2 80amp relays in parallel.

first i should do some research on how much current draw there is during cranking. I guess i only need a single 80 amp for the audio system battery. my 3 amps have ~60 amps worth of fuses.

first i should do some research on how much current draw there is during cranking.

I agree with you.

i think I could get away with wiring 2 80amp relays in parallel.

Might work... and @ $13... NOW that better price... the other was having a scary price tag... for me...

Id be careful with the ampacity
I orsered a starter and in the spec/test sheet it said about 350 amps starting max
im sure its only a second or two but that's alot of amps
especially with batteries that can do 1000 cranking amps( 30 sec worth continous) that can be dangerous

thanks for the heads up. i was looking at the switch that is in there now:

and it's rated at 250/cont 350/max. 3 would handle this with some to spare in the max dept. am i glossing over anything?

I believe that a relay that carries starting current should be capable of 1,000 amps. Cranking current for a V8 could easily be 500 amps. The thing you want is normally termed a starter solenoid -- google will find lots at "marine industrial starter solenoid". These jobs will have a pull-in current of 10A, so you may need a reasonably hefty relay just to actuate it.

As for lead-acid battery state of charge, that is a rather complex function of the battery capacity, internal resistance, current load (or charge current), voltage and temperature. If you have all of those variables, it is feasible to make a reasonable guess about SOC. Normally you do it by setting current = 0, making an assumption about the temperature, and basing your guess on the voltage.

One of my favourite references is Battery University -- http://batteryuniversity.com/learn/article/calculating_the_battery_runtime

hmm... well the cost of this build just went up!

looks like there are solutions out there that can be controlled from a microcontroller

but they aren't cheap.

the starter solenoids that i've been researching aren't rated for continuous use. i have found some that are rated at 200 amps continious and emailed the company to see if they are safe for cranking:

http://www.go2marine.com/product/86818F/continuous-duty-solenoid-6-36-volts-200-amps.html?utm_campaign=googleshopping&utm_medium=productfeed&utm_source=googlebase&WT.mc_id=gb1

this one has had two reviews online that i've found that have mentioned that they've used them while cranking:

and since my perko switch that's in there now is 350 max. i believe that this one should be safe to use even w/o the 1k amp max rating?

I don't know how much starting current changes, I have a v6 and I've been told I have a tiny starter
but if the original is rated to 350(id assumed desgined that way) then it will probably be fine
Depending on the loads your putting out perhaps a simple volt test would work, say set an alarm off if it goes below 12.2-5
if its a regular 12v battery they are nominal at 12.8 and 13.5-8 when charging
if it gets to below 12v with minamal load then definetly disconnect it, below 11.5 usually means dead ( not enough to crank) atleast from my experience with car batterys

scott_fx:
the engine is an indmar (aka chevy) v8. i agree that it needs to be heavy duty. I'm hoping that the 200 amp versions would have plenty of headroom while cranking.

I think you're out by about a factor of five. I think 500A would be a reasonable guess for the crank cold starting current, but depending on the starter motor it could be higher. In any case you should design your components to have an ample reserve capacity. A factor of two would probably be sufficient, leading to about 1000A rating.

PeterH:

scott_fx:
the engine is an indmar (aka chevy) v8. i agree that it needs to be heavy duty. I'm hoping that the 200 amp versions would have plenty of headroom while cranking.

I think you're out by about a factor of five. I think 500A would be a reasonable guess for the crank cold starting current, but depending on the starter motor it could be higher. In any case you should design your components to have an ample reserve capacity. A factor of two would probably be sufficient, leading to about 1000A rating.

even if the current manual switch i have is rated at 250A continuous and 350A max?