12v battery maintenance - temp compensated

Linear and non linear

12v battery maintenance - temp compensated

Postby Doug Coulter » Tue Feb 26, 2013 3:58 pm

This is kind of low tech for here, but also more sophisticated than just a battery or trickle charger. Due to picking up on the amateur radio stuff (and it's all 12v) I decided to do what most do who have these "mobile" units used as home stations - buy a deep cycle battery to run it off of to handle the huge peak currents during transmit, and eliminate all chance of 60hz hum. But you got to keep the battery in good condition. Normally, a big lead acid battery is NOT something you want indoors. They fume, the fumes make trouble (and smell nasty). If you cycle them a lot, even the deep cycle marine types go bad. The sealed ones cost too much, and are even more sensitive to overcharge. So, what's an engineer to do?

Well, here's what I did. There might be something like this out there already, dunno - my fancy solar stuff does this, but at zillions of times more bucks. This was 100% out of my junkbox (and I could have done it better and still not bought parts).
BatteryMaintainer.jpg
Temperature compensated battery maintainer


According to Xantrex, deep cycle lead acid batteries have a tempco of around -21mv/C. In my case, I want to hold a battery at the highest possible voltage I can without getting into gassing and wasting power, since I'm off the grid, and don't need the gas (even the new flooded marine batteries are "maintenance resistant" - hard to add water.
This requires fairly close tempco matching. Turns out silicon diodes (1n4148 in this case) have around -2mv/C themselves, and that 17 of them work out with a standard LM-350 and around 10 milliamps to just the right voltage at temperatures around room temperature. Actually, it would be perfect with 10-11 diodes and a resistor in series, instead of getting the full set voltage (minus 1.25v for the regulator) from all diodes, but this is darned close and seems to be working out in real life. The 10 diodes plus resistor might be a little trickier to tune...

Simply - I used a 100 ohm pot (ex of tektronix, this is the good stuff) and a 47 ohm resistor between output and adj terminals, then the diodes to ground (note how I sensed the ground off the negative output, not someplace else). I added a diode backwards across the lm 350 in case someone hooks up the battery before powering the unit on, to save some low current junctions in the regulator from that abuse - having to quick charge the 4700 uf filter cap. I stole the xfrmr out of a 12v halogen desk lamp fixture I'd converted to LED. I could have used a larger xfrmr, at the cost of more losses due to core excitation current, and more drop across the regulator.

This circuit as shown (cheezy radio-crap bridge rectifier has a lot of forward drop) will not make big currents into the battery till the voltage drops pretty low. For this use, that's fine. For doing higher currents or holding the battery up well above 12.6v even under load, you'd want a higher voltage (maybe 15v) transformer, and/or shottky diodes in the bridge instead. But - then it would be less efficient the other 99.99% of the time when it's just floating and the supply itself is drawing more current than the battery(!). The way it is, the supply is kept near the battery, and if the supply heats up for whatever reason - the voltage goes down...reducing the current output and the heating. So I can hold this marine (johnson controls, sold at walmart) right at 13.7 or so volts, near-zero current, no gassing, for when the station is off, but always ready to go.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: 12v battery maintenance - temp compensated

Postby johnf » Wed Feb 27, 2013 5:05 pm

Okay Doug
here is a couple of ramblings I did a a couple of years ago for solar power electric fences run from an SLA battery.
the analog version was not made but should work.
I have made several of the switching versions.
Temp coefficient is done just as you did with the diodes in the feedback loop giving 18mV /degree C which is close enough to the 20mV/degree C that litrature suggests
the extra diode in the analog one is to compensate for the series diode in the output that stops discharge when the sun dont shine
solar1.JPG


solar2.JPG
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Re: 12v battery maintenance - temp compensated

Postby johnf » Sat Mar 02, 2013 2:19 pm

Whoops
I had the mistaken impression that diodes were -6mV/C so I do not have enough tempco correction
to correct this add another 7 diodes in series and drop R1 (switching design) from 8K2 to 3K9

and Doug your design is probably bang on as from what I read on diodes from Philips is that tempco = -2.1 to -2.3 mV/K
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Re: 12v battery maintenance - temp compensated

Postby Doug Coulter » Sun Mar 03, 2013 6:35 pm

Really nice stuff, John, I bet it works great. I calculate it takes 10-11 silicon diodes (if run at low current) to get the right ratio of volts to volts/deg C for a 12v lead acid.
In my quickie design, I was after an experiment in over compensation, since this will be hooked to a large deep cycle battery that will sometimes see 30 amp loads - with a 5 amp (tops) regulator and a 1.6 amp transformer driving it. Thus, there will be times this cannot keep up with the load (key down on the transmitter) and will heat up a little. The overcompensation means that this will not even try super hard to keep up - when it heats up, it'll kind of give up till it cools back down (in this application).

To get exact compensation, you should use about 11 diodes (we are running around 10 ma on 200 ma diodes, under which condition they show about .71v forward drop).
But that doesn't get you to the nominal float voltage, so you'd have to add a series R in the diode string to make up the difference. it would need to drop about 6 x .71v at 10 ma or 4.26v. That would be roughly a 426 ohm resistor in the current setup, replacing 6 of the diodes to ground. The 100 ohm pot + 47 ohm resistor between the output and adj pin simply adjusts the current through the diode string to around 10 ma for fine tuning.

I'm going to do another for my "outback" plumbing system that also runs on 12v with a battery. That one might look more like one of your designs, since I DO want it to keep up with a 7-8 amp load (the spot sprayer pump that pressurizes the plumbing there) even if the battery is on its last legs (the current situation). I will also have a separate enable so the pump can't run when I'm away for a time - I don't want a drip in some fixture to pump my whole cistern into the drain-field (it's happened, a real PITA as it takes time or work to get that cistern full again - rain usually, or I have to haul the water up to it, 50 gal at a time).

For this little thing, I just didn't want to go all out on parts and stuff. Junkbox-only. I depend on the regulator's internal safeties to protect it, and the "sloppy drop" of the transformer to limit current into a 12v battery - it drags down to the regulator drop-out point if overloaded. A 14-15v transformer that had some snot would require more protections and maybe that current booster you show in your first design.

So far, the design as shown is working fine - the radio only draws around 1 amp on receive, and I don't talk that much (believe it or not! ;) . The thing heats up almost none at all under those conditions - the transformer makes just enough peak DC to run it - a somewhat bigger load and the transformer (after rectification) drops down below the drop out point of the regulator.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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