Poor boy's gelcell charge circuit questions, - Page 1
Forum section: electronics / projects

author: CaptDon
author_rank: Super Contributor
timestamp: March 01, 2022, 02:36:46 pm
content: I have a need for a 'closet dwelling' circuit that won't be touched by human hands for about 3 years once installed. The backup power is a 10AH 6 volt gelcell. There is a low voltage cutoff for the load at about 5.4 volts. The charge circuit is supplied with 10 volts D.C. through an 82 ohm 1 watt resistor (gives about 40ma.) and a 6.8 volt zener across the battery (Fused in case of zener short). When mains power is good there is no drain on the battery and 40ma. available as charge current until reaching the zener point at which time most of the charge current will be heat in the zener.Does it sound feasible? I have some 7.2 volt zeners but I think that is too high for float duty? The load current during power outage is only around .35a long term average and peaking at .6a when the OCXO heater is on. The heater sees a 20% duty cycle in summer and a 50% duty cycle in winter. The battery backup target is 24 hours of operation during power outages. This is to add 'dirt cheap' battery backup to an existing maritime AIS stationary system.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2022, 03:16:31 pm
content: Why not a pass regulator? They can be more accurate (1%? sharper knee?), and will dissipate less.What's hard to design about the zener is, the knee isn't abrupt, it has some curvature to it. It's 6.8V at rated current (10mA or whatever it says), but not suddenly 0mA at 6.7V or what have you. The next data point they give is the leakage, at 5V or 2V or whatever. And inbetween, it's some exponential sort of curve, but you can't really know how steep, unfortunately.On the upside, 6.8V is one of the sharper values -- zener performance peaks right around the 6.2-6.8V rating -- so it's probably quite serviceable that way. You should get more total capacity with a different design -- which, Idunno, 5%, 10%, or more? Depends on downtime between charges, diode leakage, etc. (That is to say: while it's not charging, the diode leakage is discharging, wasting some of your capacity. Another fact that works in your favor is, due to a slight electrode overpotential -- charging voltage is somewhat higher than nominal voltage. So, once charging stops, voltage relaxes down to nominal, also putting it lower on the zener curve.) Which, granted, that might not be too impressive of an improvement, so if that's good enough for you -- y'know?Another thing you don't get with a zener, is temperature coefficient. Lead acid has a negative tempco, so for a constant charging voltage, it'll be overcharged at high temperature, and undercharged at low. These mid-range zeners happen to have very low tempco -- hence they make excellent voltage references -- but that works against you a bit here. On the upside, we can potentially compensate by wiring a regular diode in series (which has a negative tempco). (Indeed these are available as standard parts, when the tempcos cancel out very closely -- I forget if this uses a 6.2 or 5.6V zener or what, plus the ~0.6V diode, but it's something around there. So, you might not have many combinations of zeners and diodes to play with.) And mind that lower voltage zeners, and regular diodes, all give a softer knee, thus incurring more leakage.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2022, 03:24:25 pm
content: If you select the 6.8V Zener on test for an appropriate float voltage for your battery and expected temperature range it could work OK. However I'd probably go for a LM317 based solution to allow faster recharging after a power outage, with a Schottky diode in series with the output to prevent power-off discharge and a PNP current sense transistor on the input sidepulling updriving a NPN to pull downADJ to limit the max. charging current. You can even add temperature correction to the voltage set divider with a thermistor in contact with the battery case.Edit: corrected current limiter

author: LaserSteve
author_rank: Super Contributor
timestamp: March 01, 2022, 06:27:26 pm
content: We have an ultracold freezer at work with two gel cells as control systems backups.. It has a continuous charge trickling in at about 10 ma. Every two years, like clockwork, first the never used cell dies, then the other, that does get a tiny. load draw once a month, fades away. Both batteries, 12v 8 AH are usually useless at that point.Every 15 minutes or so the freezer checks the batteries for a few seconds. When the alarm sounds at 12.6 volts, generally I'm replacing dead bricks.My point being, draw some serious current on the battery periodically, or when you need them, there will be nothing at all.I've had the same fault on two different brands of freezer.I'd make a serious inquiry to the US Coast Gusrd ATON lab and ask how they do it.Steve

author: Gyro
author_rank: Super Contributor
timestamp: March 01, 2022, 07:41:20 pm
content: The classic National Semiconductor LM317 Lead acid battery charger app circuit. Rs increases the output impedance of the regulator, reducing the maximum charge current (and PSU loading) when the battery is discharged (R2 needs to be lowered for float charging a 6V battery rather than 12V)....P.S. They never did fix the typo - Rs allows low charging rates with a fullydischargedbattery. It's still in the TI datasheets to this day!

author: SeanB
author_rank: Super Contributor
timestamp: March 01, 2022, 08:34:56 pm
content: Seen a later on App note with an upgraded circuit that has a few series diodes in the lower leg, to provide temperature compensation, plus a diode to isolate the output when power is removed.

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2022, 11:11:07 pm
content: Yeah, but its *horrible* as a float charger (especially if the battery has any standing load) as it cuts back the charging current well below the float voltage. To do the job half-way 'right' you need to use either an external current sense circuit (e.g. two transistors and a few resistors), or just use the LM317 for current limiting and use something else (e.g. TL431) to drag its ADJ pin low when the float voltage is reached. Muntzing it to save two jellybean BJTs and three resistors just isn't worth the risk of premature SLA failure due to undercharging.See the current through the discharge protection diodes in the attached LTspice sim of the N.I. fig.53 circuit, and the one with a BJT current limiter I proposed earlier in reply #2.N.B. I'm simming it with LT1085 regulators which are close enough to LM317 ones and avoid the need for 3rd party models.

author: CaptDon
author_rank: Super Contributor
timestamp: March 02, 2022, 03:59:49 pm
content: I went with the 6.2v zener with a standard 1N4004 in series. I did notice doing some careful measurements in the microamp range a bit of hysteresis. A slow ramp up of voltage until I begin to see 5 to 10 microamps of current flow, then I have to ramp back down about .1 volt below the point where current started to flow to establish zero current flow. Anyway, the circuit does seem to limit the float charge voltage to around 6.9 which is the suggested float voltage marked on the battery. Granted after power fail it will take a long time to recharge through the series resistor set for 40ma. average current flow, but the unit is always on and is unattended for years (unless it fails).

author: floobydust
author_rank: Super Contributor
timestamp: March 02, 2022, 05:49:56 pm
content: With ~40mA charging current, it will take many days to charge or recover.Given the cost of a battery, I would splurge and instead use an LM317 or TL431 with some extra parts. They are much more precise for charging voltage setpoint, which is crucial for long gel-cell battery life. It depends a lot on ambient temperature as well.I did a small 12V LM317 charger but it had to work from -30°C to +30°C where a constant voltage float charger is no good. Overcharges when hot out, and undercharges when cold out.I also find zeners don't last, they seem to age and go high value or open-circuit in shunt regulators that are always on 24/7.