OT - Battery care for winter or storage

The 2A rating on a lead acid battery charger is more like 2A for a discharged battery and lower current depending on the state of charge, quite different than a 2A constant current charger. So, with what I was thinking if the battery needed the charge, it would draw up to 2A from the charger, if not, it might only draw a fraction of an amp from the charger to keep itself in a good state of charge.

From what I read today, a daily charge voltage should be from 14.2V to 14.4V at 20 Deg. C with a -0.022V per Deg. C temperature coefficient. I have some PLC's around with RTD and thermocouple inputs plus analog inputs and outputs. I guess to ultimate solution would be to measure the temperature, calculate the perfect charge voltage, and apply it to a daily charge routine. I'm more likely to just switch my Schumacher 6A/2A charger to each battery for an hour a day and call it good enough. At least it would be better than no charge at all until I need to use the vehicle.

I have thought of using a diy circuit, perhaps similar to what you are thinking of. I was thinking of using an adjustable voltage regulator with a resistor inline to lower the voltage output as the current increased. I played with a circuit simulator and found a resistors that would give good current at 12V and decrease to near zero around 14.4V.

RogerN

The "2A" range on my 2/10/50A Schumacher starts at 4 - 5A and decreases as the battery comes up; it passed smoothly through 2A but didn't stay there, IIRC at cutoff the current was less than 1A.

An LM317 adjustable regulator IC is enough to make a decent trickle charger. Adjust the pot that sets it to the end voltage, the device limits the current to 1.25A max, then it falls off as the battery nears the set point. You could print the temperature compensation chart and tape it to the box.

This is a commercial version with voltage and current meters:

The one-turn voltage adjustment is tricky but good enough for lead- acids, though you'll need a separate voltmeter with more resolution

jsw

/ /Good idea. / /I would just avoid running extra long leads on the low voltage side /whenever possible...it is just too easy for voltage loss to cause you /problems especially now with smart chargers monitoring the battery's /voltage.

Some of the chargers description described as charging and then monitoring the voltage, when it falls below a certain point it charges again. This had me thinking that it monitors voltage with almost no current flow. I'd guess it also monitors voltage during charge with current flow. I thought if I were brewing my own, I would use shielded 4 conductor, 2 wires for charge and 2 others for measuring voltage.

/I have found that I need a "dumb" charger to start batteries who have /been seriously discharged and then allowing the smart charger to /finish...again that little chip in the smart charger being a bit too /"smart".

Occasionaly I have to use a good battery in parallel with the discharged battery to get my "smart" charger to work.

/Idealy you should monitor your batteries with a hydrometer to /determine the acid/water ratio to know when you are charging at the /optimal rate and time. / /TMT

I would think that would be the best way to get the ultimate charge without overcharging.

I've played with an adjustable regulator circuit in Electronics Workbench simulator. I used the typical voltage divider circuit to set the open circuit voltage and used an inline resistor so that the voltage goes down as the current goes up. I got an open circuit voltage of 14.201V, .19A at 14V,

2A at 13V.

RogerN

None personally, but I met a merchant marine sailor that kept a car in storage while he was at sea and used one of these and he seemed to be pleased that he could start his car and go after leaving it for months at a time.

Seems to me it might be worth a shot for the OP to try as draping wires all over the place or lugging batteries in and out of vehicles in the freezing cold can't be fun.

Kelvin connections won't help you with lead-acids. They charge to an artificially high voltage due to "surface charge", which takes time to dissipate. This article says 4 - 8 hours:

Agreed. I need the homebrew lab-supply chargers for older batteries whose cells have become unequal or sulphated.

You don't need the series resistor, the exact shape of the knee (constant-current to voltage transition) doesn't matter.

jsw

/ /The "2A" range on my 2/10/50A Schumacher starts at 4 - 5A and /decreases as the battery comes up; it passed smoothly through 2A but /didn't stay there, IIRC at cutoff the current was less than 1A. / /An LM317 adjustable regulator IC is enough to make a decent trickle /charger. Adjust the pot that sets it to the end voltage, the device /limits the current to 1.25A max, then it falls off as the battery /nears the set point. You could print the temperature compensation /chart and tape it to the box. / /This is a commercial version with voltage and current meters: /

one-turn voltage adjustment is tricky but good enough for lead- /acids, though you'll need a separate voltmeter with more resolution / /jsw

That would be interesting, I could send a higher DC voltage to the vehicle and have the voltage regulator on board. It would be neat if I could get temperature compensation close with a PT100 RTD or similar, I guess I'd just have to calculate the temperature coefficient of the platinum versus -0.022V/deg C for the battery. Anyway, if I could get the circuit right, I could power it a couple of hours a day and keep the battery topped off. I would think that would do at least as good as an automotive alternator/regulator does.

RogerN

The LM317 regulates current instead of voltage if you tap off the ADJ terminal. It tries to force the voltage between OUT and ADJ to be

1.25V, so a 12.5 ohm resistor between them will set a constant current of 100mA, for example. Make the current a little higher than the measured parasitic drain current of the car's electronics and don't worry about voltage or temperature.

I don't do this to in-vehicle batteries because anything left connected to the positive terminal is a possible short-circuit fire hazard if it fails.

jsw

/ /Kelvin connections won't help you with lead-acids. They charge to an /artificially high voltage due to "surface charge", which takes time to /dissipate. This article says 4 - 8 hours: /

I was thinking of charging to a 14.2-14.4 charge voltage, the voltage would be sensed by 2 wires with very low current. The reason is because if I ran a wire from my house to my batteries in vehicles, the wire would drop some voltage on the current carrying charge lines. The idea is the same as using a 3 or 4 wire connection to a platinum RTD. The current carrying leads of an RTD only carry 1mA but that's enough error to throw the reading off. If I did use something like a LM317 regulator I could adjust it for 14.4V and as the battery charged, and the current decreased, the voltage drop in the wire would also decrease.

/ /Agreed. I need the homebrew lab-supply chargers for older batteries /whose cells have become unequal or sulphated. /

/ /You don't need the series resistor, the exact shape of the knee /(constant-current to voltage transition) doesn't matter. /

/jsw

By using a series resistor I can make the supply current go to zero as the voltage approaches full charge voltage. This gives a power source that is limited in both voltage and current.

RogerN

If you set the circuit up and measure it you'll see that the current decreases naturally as the battery voltage nears the open-circuit output of the supply. The $4 HF meters are good enough for this, barely, and cheap enough to risk frying from an accident. The meter lead and shunt resistance on the 10A setting will round off the current/voltage transition knee.

Kelvin connections won't account for the battery's internal resistance or surface charge effects. Some smart chargers periodically shut off the current, wait a bit, then measure the battery voltage after it's had time to settle.

jsw

I had a Horrid Fright float charger. Boiled my marine battery dry, and killed it.