Deep cycle lead acid battery monitor

[large_ghostman]

as part of my boat project i want to have a battery monitor, but i have a few questions regarding the battery.
we have a 12v leisure deep cycle battery that powers the boat electrics and provides power to start the boat, what we dont have at the moment is anyway of knowing when the battery is getting too low to restart the boat, this normally happens if we are night fishing and anchored up with the engine off and all electrics on for long periods. so anyway i was thinking of a simple row of leds with say 5 green and 3 red that show how much juice is left in the battery (number of leds is just a random number i picked), so what is the best way of doing this? can it be done by simply measuring how much voltage is in the battery? and when it drops to a certain level flash a warning? if so what kind of voltage says the battery is getting low? i dont need any help with the code for now but i am not sure how to actualy tell if the battery is getting low

LG

 

[be80be]

The voltage will drop if loaded but as soon as the load is removed the voltage will go back up.
To test it you need a load on the battery.
Most testers that can tell you if the battery is strong enough to start a motor put a heavey load on the battery to test with.

 

[alec_t]

Google 'battery gas gauge'. There are ICs for this.

 

[large_ghostman]

i have had a google but most the ic's seem to be for li ion batts?? i will do some more digging in the morning now i know what i am looking for. if i cant find one what kind of load would i need? i have some 25-50W resistors but they are about 15 ohms i think

 

[jpanhalt]

Here's an article on testing lead-acid battery capacity: **broken link removed**. Are you sure you need to do that? Does your boat motor also have a pull starter? Why not just take notes and use a little common sense. Then, if the battery can't crank, use the pull starter.

One problem with your question is the unknown of how easily the motor starts under the current weather conditions. Some motors will start if a spark is applied anywhere on the compression stroke. Others will need to be cranked for something like 30 seconds, and there is everything in between. Does your motor always start on the first "flip"? Humidity and temperature will affect how a motor starts.

John

 

[large_ghostman]

the motor is a 2stroke 60hp it dosnt have a pull starter, starts pretty easy 2-3 cranks. i can see how difficult it is now to get a monitor to do what i want, i assumed i just needed to watch the voltage but apparently not. back to the drawing board!

 

[jpanhalt]

I haven't seen a big 2-stroker like that in a long time. The last one I saw was in an old Subaru and had 3 cylinders.

I would be quite concerned about not having a way to start that motor in a emergency. Can you carry a second, hand-start emergency motor -- something like a trolling motor? Would a small petrol powered generator for recharging the battery or running your on-board power be something to consider? Or, maybe just carry two batteries. One could be just a back-up for starting and would not need to be deep cycle.

John

 

[large_ghostman]

we have a 4 stroke 10hp aux motor so we always have a way to get home, the battery never gets low enough that it wont start the motor but sometimes its low so the motor only just cranks, what i wanted was a way of telling when that point was close so we could start it to recharge the battery, the 4 stroke aux motor is a pull start one so dosnt need a battery. i am now thinking of monitoring how much we are drawing from the battery and doing it on a time based method so say deducting how many hours we have used say 4 amps from the overall amp hour rating, not accurate i know but maybe a starting place, and monitoring how many amps we are drawing will lets us know when we get a short, sometimes when salt water gets into something we find its shorting the battery out. the battery is replaced every year BTW

 

[nickelflippr]

For my camper/RV battery monitor I used 10 leds for each 10% of battery capacity. The battery capacity was determined from the manufacturers data sheet. So for me: 6 green leds for 100%-40% , 3 yellow 30%-10, and 1 red <10%. Some might say don't discharge below 50% for best battery life.

Also used a TL431 4.096V precision voltage reference for the ADC Vref+, for a 4mv/bit accuracy. That kind of accuracy really isn't needed, as all you are looking for is a relative reading (i.e. 3 yellows still starts?, 1 yellow doesn't?). So a newer chip with a built in voltage reference should be fine. Put a 4:1 voltage divider on the battery to adc input and you should be good to go.

 

[Nigel Goodwin]

What you could try, and it would make a nice little project - make a data logger - and record the battery voltage when you're out in the boat.

Then you can compare the battery voltage with how well it turns the engine over.

By using the data you collect, you could use the same data logger hardware to monitor the battery and display a warning when it drops below the level determined in the testing.

 

[be80be]

I'd save last years battery and charge it up for the ACC and not use the starting battery. I always used two on a boat one for lights and stuff, and one for starting i hate pulling a rope and i bet if you pull the cover off that 60 there's a place at the top that you can wrap one and start it but pulling the rope isn't easy.

The flywheel on most are like this those three hook looking things hold a rope.
figure you may need that some day if you get stuck out on the lake.

Oh The data logging is a great idea and then let it light the leds as was posted using the logged data. Great idea Nigel Goodwin

 

[large_ghostman]

great idea's guys. so just to recap i can monitor the voltage? what kind of voltage should i see froma fully charged battery? i only ask so i make sure i dont blow the pic ADC by not having a big enough divider.
a data logger is a ace idea of course it needs feature creep like rs232 to laptop to download and i could get a chance to try and log to a SD card!! something ive wanted to do for ages.
we only use one battery because of trying to keep weight down??? not sure the other reason now but there is probably a good one i will ask dad. as for a rope thing on top of the engine there is one ive looked, but dad said it was a ***** to use and in a bit of a swell he would rather jump in and swim home lol, actually his arm strength is still weak from the stroke hence why we have a small 4 stroke that a easy pull start that we take with us. i will do a circuit diagram tomorrow and plan out how to do it. great project especially the data logging part if i monitor how many Amps we are pulling out of it etc then i should be able to spot when a new battery is needed!

 

[Nigel Goodwin]

great idea's guys. so just to recap i can monitor the voltage? what kind of voltage should i see froma fully charged battery? i only ask so i make sure i dont blow the pic ADC by not having a big enough divider.

A car battery is nominally 13.8V, so a voltmeter needs to read to 15/16V or so.

But!! - a good range for a PIC would be 20.48V FSD as it makes for nice simple maths, and gives 20mV resolution.

 

[large_ghostman]

can you explain the difference between or rather the reason why the voltage is 13.8 but shows on a meter as 15??. 20mV is way better than i was hoping to get!!.

 

[large_ghostman]

so bottom of divider is 20K Ohms and top of divider is 61.9 Ohms so if i use a 100 ohm quality trimmer i can tweak it to get it right ??? or am i totaly off??

 

[KeepItSimpleStupid]

LG:

Could this work: https://www.ti.com/product/bq34z110?DCMP=hpa-pmp-bq34z110-en&HQS=bq34z110-pr

 

[large_ghostman]

now that looks like a nifty chip! only thing worries me is some the wording in the data sheet makes me think you need a fairly high min draw (32A) for it to work, but i might be wrong the way its worded isnt clear to me. only other thing i need to watch is 4.5v max input so i will need to make sure I2C and pic is running at 4.5v. but overall has loads of features and should work really well with a pic, what does every one else think?

 

[large_ghostman]

had a read again and i was indeed probably wrong so before i do too much more i will go away and read the datasheet (takes me a while as i dont always understand what they are saying), then i will be back hopefully with a diagram, with tax will be around £6 for the chip but seems well worth it considering all the stuff it looks like it can do, shame i cant get TI samples in the uk
going to read up on SD card for logging as well, i dont think i will need FAT file system as all i want to do is log from pic and when it comes to downloading the log i can get the pic to read it and send down RS232 to pc/laptop to something like hyperterminal (i dont use that but cant remember the name of one i use), and from there into excell simples lmao . battery state from chip output to leds near instrument panel and more detailed data inside cabin on lcd, sound good??? or barking?

 

[be80be]

That chip looked really good I was thinking they only made for laptop batterys.
But that one is for Lead Acid. Nice find.

 

[Nigel Goodwin]

can you explain the difference between or rather the reason why the voltage is 13.8 but shows on a meter as 15??. 20mV is way better than i was hoping to get!!.

That's not what I meant

As the nominal voltage is 13.8V, the meter needs to read a little higher than that - hence 15/16V - but for a 10 bit converter making it read 20.48V makes more sense.

Your divider values seem massively off?, what is your reference voltage?.