Deep cycle lead acid battery monitor

[large_ghostman]

lol sorry nigel i misunderstood what you were meant! i was using 20.48v as reference, and they were what i came up with for the voltage divider. i will go have another try maths isnt my best point but practice makes perfect as they say.

 

[large_ghostman]

ok i did make foof up! top resistor 1k and bottom a trimmer to make 323R should give me exactly 5V from 20.48V input (i think), also if i am correct. then with that divider i should get a divider output of 3.37V from 13.8V input

 

[be80be]

I would base it around 10 K use a 10k pot at top and 1 k at bottom set the pot for 9.1 k and use 2.048 for the Vref.

 

[large_ghostman]

do i use a pot for the vref? to give that value?

 

[large_ghostman]

also i chose the values to keep within the 2.5k resistance the data sheet said was best but i dont spose in something like this it matters?? i assume there value is there to give best speed? and i dont need very fast sample times is that correct? sorry if i sound stupid i am just trying to make sure i fully understand it all

 

[be80be]

Some chips have built in software settable Vref
Using the (1.024V/2.048V/2.096V Fixed Voltage Reference) Is on some chips .
The reason I would use a 10k divider it makes 1.5mA across it

 

[large_ghostman]

Some chips have built in software settable Vref

i had totally forgot that, i will double check but i am pretty sure the current chip i am using 45k22 has it!
thanks for the heads up i will go check the sheet to make sure


edit hmmm i dont think it does, sheet indicates ADCON1 for setting ref but dosnt say about software selectable maybe it was in the other sheet for 18f4685 (the chip i normally like). i spose i can always use a good multi turn trim pot to set the ref on the external ref pin if needed

 

[large_ghostman]

dosnt look like its on that one either but i am sure i have seen that setting you mean in a datasheet but cant remember the chip! anyway for now untill i unpack everything the chip i have is the 18f45k22.

 

[be80be]

Using 20.48 volt divider and 2.048 Vref makes the math easy on the pic your just moving the (.)

The 18f45k22 has it look on page 304
FVR BUF2 (1.024V/2.048V/2.096V Volt Fixed Voltage Reference)

And how to set it on 344 of the datasheet.

 

[large_ghostman]

well spotted burt! totally missed it, i must have seen it when i was reading about the CTMU when i as thinking of doing cap switches for the boat controls. time for bed for me i start new school tomorrow, but will be back on it when i get home, thanks for the help really looking forward to this project

 

[bryan1]

Hi LG,
It's been ages since I did any electronics but how about using a LM3814(sp) led chip so the led's can show the voltage, that would make for an easy visual indicator and no micro needed. The unit could easily powered by some nimh cells and a second chip could monitor the battery voltage so one will know the nimh batteries will be upto powering the fishing trip for the indicator circuit.

Cheers Bryan

 

[lipschutz]

why not try an lm3914? it could solve your problem right away.

 

[ghostman11]

hi guys, there is no real rush for the project as it's getting to that time for putting the boat away for winter. actually this year it's having a refit , as for a non micro solution...............not a chance lol LG has his mind set on data logging etc, plus i think there is slightly more to it than measuring the voltage hence the chip for monitoring it all and passing data over I2C.
still this part of it is upto LG i will leave it to him! my job is to strip the boat out maybe repaint as well. i have a new echo sounder to install and GPS plotter also time to upgrade the VHF the old one is a bit past it.
i might get a few more trips out in it before we put it in the workshop depends on the weather. i would also like a way to monitor charging current as i am not sure the main engine is doing a good job on recharging.
one other question as its a new battery do i keep it on trickle charge over winter? it was alot of money and i dont want to ruin it before next season!

 

[KeepItSimpleStupid]

What you need to know looks like it's here: (thanks John -japanhalt ) for your recent links)

It really looks like the problem of what the charger considers "float".
You don;t want the battery to loose water and you don't want to undercharge either. If it does, then you may want to find a better charger OR have a way of cycling the charger to turn on every 2-3 days.

I would, at least, trickle charge, take a specific gravity readings every two weeks, measure the open circuit voltage and SG. Then see what you have to do based on the results.

 

[jpanhalt]

I agree. It is said that a little over charging (gassing) is better than undercharging. The latter can lead to irreversible (IMHO) sulfation.
John

 

[ghostman11]

sorry guys the new software failed to tell me of a update to this thread! charging isnt a problem as the engine charges the battery when its running and over winter i stick it on trickle charge, main problem is when actually fishing we sit with engine off for a few hours so wanted to know when the voltage had dropped to a point that we needed to start the engine to charge it again.
but mostly i think it's a LG project, being a boat i have over-sized the battery by a fair amount!! so in reality it should run all the electronics (FF etc) all day and all night and still have plenty of grunt to start the boat. i get a new large deep cycle battery every year for the boat and this year went for a monster one lol

 

[MikeMl]

My boat has a "house battery" to run lights and toys, and a "starting battery" to crank the V-8.

 

[ghostman11]

maybe during the refit this winter i will enlarge the battery house and put two in. whatever i do isnt going to stop LG doing a project

 

[dr pepper]

You can approximate the capacity left in a battery by voltage, but surface charge and a load of other effects can be misleading, the only way to be sure is to measure the amp-hour charge and discharge, and then you have to offset that with temperature and the age of the battery.
You'd be better with a small battery for non essentials and another smallish battery to start the engine.

 

[ghostman11]

ok how about another approach. at what point should a battery be recharged? ie. should it not be allowed to go below 11v or 10v or is each different?