Super little voltage monitor circuit for Mosfet PWM control... (well, I think so :) ) - Page 2


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  1. #16
    Kapitan
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Hold the bus on the fet driver Steve, I have hit a small issue that will need a different driver chip, This has a low voltage cut out at around 12.5 volts so is not the way to go, it was ok on a fully charged battery as a test, but not if it is discharged below about 12.5..
    My fault for not reading the datsheet properly ! ...
    Will report back...
    Joe.

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  • #17
    Senior Member chuckieboy's Avatar
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Hi Joe thanks for the info, I was going to order some but never got around to it, it's a good job you said becasue my apps are 6volts even though it's a 24V system the batteries have to be tested indvidual.

    Thanks
    Steve

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  • #18
    Kapitan
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    No worries Steve,
    how exactly do you want to test them, ? then I can add it into my thought process as mine are all 6V units in series parallel.
    Where will the power come from for testing, ?
    Give me a brief run down of what you need and I will look into it...
    Joe.

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  • #19
    Senior Member chuckieboy's Avatar
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Joe The batteries are fitted to a mchine that runs on 24V-48V(T105,T125,T145,L16G), In the past we would change the whole pack and the others would go in the srcap bin, So I carried out some inverstigation and found the Alpha-bat battery tester which only did one 6 or 12V battery but very exspensive. So I set about designing my own, I use 4 Mosfet's with 4 .11R resistors use PWM to limit the current to 30amps, The PWM adjusts it's self to matian 30amps then the peurket's factor to calculate the reamining AH(capicity)on runtime, anything below 60% is scraped and above is put back in service but on smaller machines which draw less current and we carried out some tests with data logger's to see how they performed and we found that they lasted all day and so far we have had no reported breakdowns with the customer saying the machine keeps going flat, out of 100 batteries I have saved 60 of them, Has you know how much the T105's cost. also been wired in series does not help to test them in series becasue if one battery fails the rest will show poor, So that's why I only do them has a 6V battery but test 4 in one go, Only time will tell how much longer they be in service for so far they have done 6months work. But the way I drive the mosfet's is a bit hit and miss and I want to design a newer better version, but I was asked about could I get it to charge the batteris first then carry out the test and then recharge them which would save man hours, Like wiring them up in series charging them then discoonect them test them the reconnect to charge. I know that I will have to have 4 transformers and controller's etc, but it's worth looking into, I've been pondering over the idea for ages but seeing how easy it is (well you make it sound so easy) it's got me hooked again, If I can't do the tester it I still can make a workshop chager out of it and most important thing is a charger for my caravn battery for over the winter.
    sorry if I've gone slightly of track but things like this I really enjoy, Has I worked for a battery charger company for 10 years.
    Steve

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  • #20
    Kapitan
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Hi Steve, I have had a similar idea for a while to test my main boat battery bank (all T105's) - they have been in for 5 years, but appear fine. I have not done a controlled discharge on them yet and the idea was, as you mention to use good ol' Peurket with an exponent of 1.3 (Unless you have a better exponent for the Trojan's ) )? .. to calc the theoretical Ah rating left. a 30A discharge would be a good capacity re time to use as well, Just refreshed my memory with the T105 datasheet and they DO quote a 100h 20h and 5h capcity in Ah so it is possible to back engineer the exponent. !..
    Also, what cut off voltage are you using, or are you taking them right down to 1.75V per cell ?
    The reason I have never bothered, is time ... so automatic would be good. I could manually disconnect sections from the main bank to test..
    With Rick and Wastrix's help I should have the fet side sorted with the new drivers, should get them here in a week or so. the programming so far has been simple. I cant see any issues adding Peurket to the system - a simple 2 line LCD would sort it.. I was also looking with interest at the ACS714 Hall effect Current sensor or similar for an autopilot driver unit.. so that would be good to incorporate.
    The only difficulty is ensuring the batteries are 100% charged, the discharge and Ah calc is easy. but with adequate Bulk, Absorbsion and then a good float, all should be ok.
    I wish someone would invent a small electronic specific gravity tester that could be fitted to each cell !, that would be good...
    Do you ever do a desulphation high voltage charge on them ?, just wondered...

    (ps, what do you use as a load ?) that is always a problem on the boat...
    In fact Steve, you are giving ME ideas now I want one myself lol, it would be good to use every year. and also useful to test other boats batteries.
    Will definitely work on that one. I will send you a pm with my email too...

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  • #21
    Senior Member chuckieboy's Avatar
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Hi Joe
    With your batteries been fitted in a boat and me not knowing a lot about boats I would say the current draw is not massive and continous like it is in our machines they get abused been over discharged and left flat. We did try a chemical that breaks down the sulphation it the major draw backs with it was the smell it gives off and the time it took, I give a bit of credit it did regain some capacity, the test was carreid out just on the battery as is the recorded capacity was 67% then added the desulphation stuff discharged AH went to 71% carried out another 3 tests and gain 83%AH but this took 4 to 5days to carry out these tests, This was just to prove the sulphation chemical. but the cost of time ETC ETC which then was close enough to a new battery so we never carreid it on. I have a desulphation unit on test at the moment which an electronic unit you leave permanently connected providing that you charge the batteries once in a while, They where mainly used and designed for boats and that, I will find you the link out so you can have a look.

    I will go through my stuff and find you out all the calculations and the info I found. I found with the peukert's factor the real results did not match the calculations, I've collected all the info from the alpha-bat tester and a very good contact I have. I'll send you some info over a bit later on.
    Last edited by chuckieboy; 24th July 2010 at 11:20.
    Steve

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  • #22
    Kapitan
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    Default Re: Super little voltage monitor circuit for Mosfet PWM control... (well, I think so

    Hiya Steve, great stuff
    I have been looking back at my data too, for the T105 units, based on manufacturers spec, using a 20h of 225Ah and a 5h of 185 Ah the Peukert's Exponent is around 1.16. Many often quote 1.3 for this type of battery which is incorrect.
    The only thing that never ties up is the 100h rate that trojan quote... never did work that one out...
    But the 1.16 should prove accurate at normal discharge rates.
    I will do the math in a spreadsheet first, then transfer it to the PIC... I will also take one of my batteries out and do some physical tests to compare...

    It looks like a 30A discharge on a T105 should work out at around 6.37 hours, giving a total Ah capacity at that rate of 191.19, and a Peukert corrected amp value of 35.3, so working back, 35.3 * 6.37 gives us our original quoted 225 Ah..

    Yes, it will be interesting to put the whole equation into the pic...

    Here are links to two small XLS files from a reliable source.

    http://www.smartgauge.co.uk/calcs/peukert_2.xls

    http://www.smartgauge.co.uk/calcs/peukert.xls

    Joe









    Last edited by Kapitan; 24th July 2010 at 12:08.

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