Car battery maintainer - please review my circuit - Page 1
Forum section: electronics / projects

author: TwistedTransistor
author_rank: Contributor
timestamp: December 18, 2023, 02:55:24 am
content: HiI have a project car that tends to sit on the driveway an awful lot nowadays and I'm constantly struggling to keep the battery charged. I've had to replace it twice in as many years.So I decided to design a battery maintainer to keep it perpetually on a float charge.I want something that can be attached and left without having to disconnect the battery from the car.I want something that can sit in the garage, with cables that can run out to the car on the driveway. Approx 5 meters.Commercial products are available but they tend to be based on switching power supplies and I simply don't trust some cheap switcher built down to a price not to blow up my expensive ECU.I designed the attached circuit and thought I would see if anyone would be kind enough to do a peer review before I sent the PCB out.It's a linear power supply and probably grossly over-engineered for what it is. I have tried to allow at least 50% design margin on all components to make it as robust as possible.I also wanted something accurate and low-drift that I did not need to trim or adjust.Temperature compensation is provided by a remote probe that will be placed next to the battery.Any comments much appreciated.

author: TwistedTransistor
author_rank: Contributor
timestamp: December 18, 2023, 03:03:54 am
content: Compensation was determined by some prototyping.Closed-loop frequency responses attached at 500R load (25mA) and 25R load (500mA).

author: TwistedTransistor
author_rank: Contributor
timestamp: December 18, 2023, 03:07:31 am
content: The unit is designed to operate at temperatures between -10 and +40 degrees Celsius.Temperature compensation curve attached.

author: moffy
author_rank: Super Contributor
timestamp: December 18, 2023, 03:09:08 am
content: Very nice, but it looks overly complicated for the intended purpose, which is basically a fixed voltage current limited supply. You could achieve almost all that with an LM317 and a series diode and maybe a series resistor. I would certainly drop the input voltage to your regulator to between 18V-20V to limit power dissipation, a lower input voltage also means a lower voltage in the case of a fault condition.

author: Shonky
author_rank: Frequent Contributor
timestamp: December 18, 2023, 03:12:02 am
content: Is floating it to 13.55V constantly the best for a vehicle battery?Ctek for example only pulse it occasionally to keep it charged. In their words "Maintaining the battery at 95–100 % capacity. The charger monitors the battery voltage and gives a pulse when necessary to keep the battery fully charged"I see mine pulse about once a day after the initial charge and later around every1.5 hours after it's been sitting for a few days in the maintain modeYes it does seem over complicated.

author: Whales
author_rank: Super Contributor
timestamp: December 18, 2023, 03:16:54 am
content: You are right when you say this is over-engineeredMy understanding is that a resistor trickle charger is enough for lead acids, but given that you mention an expensive ECU and batteries only lasting weeks: your cars must have notable quiescent current draws?This design looks like it can supply a lot more current than just a trickle charge. What's your target for current delivery? I see a 5A (F4) fuse, but then a 1.5A fuse in series with it (F3) and a transistor labelled as 1A current limit (Q3 + R25).I'd move the 1.5A fuse right to the output pin (where the 5A fuse is) and remove the 5A fuse. EDIT: If you're worried about the output clamp blowing the transistors then lower the fuse on the secondary of the power supply to something like 1.5A too. I don't think the rest of your circuit will pull another 0.5A on top of the charging current (assuming the charging current is 1A).Your 1K minimum load resistor (R36) might flatten your battery if you hook this up and then it loses power (or if a fuse blows inside it). It would be better to put that on the power-supply side of the diode so that it can't drain the battery.I do not believe the LM358 can deal with voltages within about 2V or so of the positive supply voltage. There might be ways that your circuit could possibly end up doing that, even if just for short moments. Could these opamps then lock up with phase inversion (or is that not something that LM358's do?).You're tying the thermistor input up to VREF (10V) which is good, that way the opamps U3B won't accidentally get the full VCC as input.Your thermistor model (J5) looks plastic coated (isolated) which is good. You wouldn't want it shorting against bodywork. I'd actually put a fuse on both legs of it, just in case.You don't seem to have any protection or sanity checks on the voltage target reference generated by the thermistor circuitry (TP17). If something goes wrong there then you battery might end up undercharged or overcharged. Might be worth thinking through all the possible failure modes for this part of the circuit.EDIT: What is the motivation of the filter (R32, C11, C4) before the main voltage control opamp (U2A)?Also I spot a 5.6V zener in its feedback loop, but its non-inverting input is being fed an exact desired battery voltage from the tempcomp circuity, so I feel like I might not be understanding something.

author: TwistedTransistor
author_rank: Contributor
timestamp: December 19, 2023, 12:54:05 am
content: Quote from: moffy on December 18, 2023, 03:09:08 amI did start off with a more minimal design based on an IC regulator but once I decided I needed temperature compensation it seemed a more natural option to simply beef up the output of an opamp with a pass transistor.The optimal float voltage varies by as much 1V over the full operational temperature range (-10 to 40 Celsius, typical UK climate). Better plot of the designed output voltage vs temperature attached.

author: TwistedTransistor
author_rank: Contributor
timestamp: December 19, 2023, 12:58:21 am
content: Quote from: Shonky on December 18, 2023, 03:12:02 amI'm by no means an expert but from my research a constant float voltage is the "classical" method of maintaining a lead acid battery at full charge.Some manufacturers seem to do a daily charge like you say but I couldn't find any consensus on whether there is any real advantage to that.Ultimately I decided on the classical method as it was simpler and avoided any microcontrollers/software.

author: TwistedTransistor
author_rank: Contributor
timestamp: December 19, 2023, 01:18:15 am
content: Quote from: Whales on December 18, 2023, 03:16:54 amQuiescent current from the car is about 70mA. It takes a couple of months to go flat. Yeah, I don't drive it very much at the moment!I suppose I should really have called this a "Car Power Supply", since that's what it's really doing. It's not just charging the battery, it's effectively powering the car.Charging a lead acid is easy, but charging a lead acid that is attached to expensive car electronics is where my paranoia sets in, and where the circuit complexity arises!Quote from: Whales on December 18, 2023, 03:16:54 amUnder normal conditions, the circuit is designed to limit the current at 1A.The 1.25A fuse is designed to work in conjunction with the over-voltage crowbar circuit. The crowbar should only ever activate if there's a catastrophic failure of the pass transistors and the output voltage rises above 18V.The crowbar shorts all the current to ground and the fuse should pop to protect the car. It's a fast blow fuse whereas the others are all time delay type fuses.There's that paranoia I was talking about.The 5A fuse on the output is designed to protect against connecting to the battery in the reverse polarity (in conjunction with diode D11) and against any component failures that short circuits directly across the battery.It's quite a high rating because I wanted to minimise the voltage dropped in the fuse during normal operation. There's a bit of a trade-off.Quote from: Whales on December 18, 2023, 03:16:54 amThis is a good point. The min load resistor is needed to ensure unconditional stability of the regulator and placing it the other side of the diode defeats that purpose.It is one thing I don't like about the design but I am not sure what can be done. Ultimately I decided to live with it since it will only pull an extra 13mA from the battery which is only going to increase total quiescent current by about 20%.If the output fuse F4 has popped then the under-voltage lockout comparator U4A should change state and the 'active' LED will go out. So there is at least some visual notification.Quote from: Whales on December 18, 2023, 03:16:54 amAnother very good point. The comparitor U4B should trip if the voltage demand exceeds 14.4V (approx gassing voltage). This can happen if, for example, the temperature probe gets disconnected and the temperature input gets pulled to 10V by R14. Under-voltage conditions I am much less worried about since the circuit will be protected by diodes D10 and D5 (belt and braces!)Quote from: Whales on December 18, 2023, 03:16:54 amIt provides some phase advance in the feedback signal. Again I found it necessary to ensure the regulator was unconditionally stable during prototyping. I'll check and tweek the values once I have the PCB.Quote from: Whales on December 18, 2023, 03:16:54 amThe Zener prevents the voltage control opamp U2A from winding up to Vcc when it is out of control of the output voltage, for example when in current limit.The opamp output is effectively constrained to within +5.6V of the feedback voltage. This prevents a big overshoot when voltage regulation is restored, which could trip the over-voltage crowbar.

author: Whales
author_rank: Super Contributor
timestamp: December 19, 2023, 07:18:49 am
content: > The min load resistor is needed to ensure unconditional stability of the regulator and placing it the other side of the diode defeats that purpose.Doh, didn't think of that.> [5A fuse] It's quite a high rating because I wanted to minimise the voltage dropped in the fuse during normal operation. There's a bit of a trade-off.There might be a way of keeping things stable, getting the right DC at the output, moving the load resistor to the left of the diode and having only a single (1.5A) output fuse. Checkout the feedback path I've drawn onto the attached, it's a vague description of a solution where HF feedback comes early (well before the caps & sources of instability) whilst LF feedback (including DC) comes from the output.100k might be a bit too big (opamp input bias currents might be bad) but hopefully you get the idea. A TVS diode somewhere to prevent frying the opamp when people touch the output lead would also be good.