Car battery charger

[Theo]

I think I might need a new car battery charger. However I'm failing to find anything suitable and wonder if the team can recommend any models.

Requirement #1 is to be able to charge flat batteries. My boot lid sometimes doesn't latch properly and leaves a dim little light on - which if you don't drive for a week or two empties the battery. My current 1980s dumb-charger can't handle delivering current into a low voltage battery for more than a few seconds before the thermal trip goes[1]. I've seen smart chargers say 'we handle flat batteries >7.5V' which is no use when the voltage drops below that.[2]

I would expect that mode to be a constant current mode, although they could pulse or do other fancy things. But it should be able to deliver it continuously without tripping out.

Requirement #2 is a display of voltage and current so I can see what's going on. Little LEDs saying 'half' or 'full' do not cut it.

Nice-to-have #3 is a means of terminating charge when it's full, but I could live without that.

Are there any 'smart' chargers that aren't also 'too clever for their own good' chargers?

Thanks Theo

[1] Instead I've been using a old 32V 2A bench PSU in constant current mode, but 2A isn't very much and the case gets very hot (suspect a linear PSU). It also isn't great to leave in the engine bay as it isn't weatherproofed in any way. [2] You might say the battery has had it by this point, but it's an AGM battery in a hybrid so it lives an easy life. It only needs to provide 20A for a few seconds to open contactors, it isn't used to start the car. I might look at a LiFePO4 SLA-replacement next time.

[Scott]

Could he rewire the boot light through the ignition circuit so there is no need to remember to flip the switch?

[Theo]

There's already two switches, one on the tailgate (which doesn't help if it hasn't shut properly) and one on the bulb itself. But I leave the bulb on because I want to see when I open the boot in the dark.

I tend not to have the engine running when the boot is open.

I keep meaning to fit an LED, which will reduce the drain from 5W to 0.5W. But it doesn't address the original question.

Theo

[Brian Reay]

If you repeatedly ?totally? discharge your car battery, you will ruin it- they aren?t designed to be totally discharged, and certainly not left in that state.

You need to either find out why the boot isn?t latching and fix it or at least check it is closed correctly.

As for chargers, you may find one with meters etc but I expect it will be expensive, unless it is an old (second hand) simple dumb charger. Modern ones tend to have lcd or led displays and have smart charge capabilities. Aldi and Lidl have them on offer every few months- each do versions of the CTek (or Ctech?) charger which are pretty good. I have an Aldi one and I bought my son in law a Lidl one. They are essentially the same, just branded differently. Have a look on YouTube.

Neither have proper meters. They attempt to emulate a meter on the LCD.

They do charge very low batteries, the early ones didn?t. There is a video on YouTube covering this.

[tabbypurr]

I'd replace the light with an LED first.

Re chargers, if they're smart they won't charge a flat battery. It helps to understand why: checking a battery is connected & correctly polarised is a safety & liability issue, so they're never going to decide to skip that in today's world. Your only solution is a separate charger that can handle th e initial charge to get it upto 10v or whatever the smart charger requires.

Ways to do that: get an old dumb charger, plenty at car boots, most don't trip

- add a series 12v bulb to one that does trip

- live with your linear psu

- use a charging wallwart so it limits rather than trips

- initial charge from another lead acid

As said, repeated discharge will kill your lead acid very early.

NT

[Paul]

There were two switches in the trunk of my previous car.

A three position slide switch. A trunk-closed switch.

The three position switch had:

Always ON Gated by trunk-closed switch Always OFF

as the three positions. If you wanted to lock an elf in the trunk and have the light ON, you could do that.

You should check and make sure your "option" switch is set in the best position.

One of my door switches on the old car was intermittent, but, the system is normally open, and the intermittent switch means it doesn't come on when the door is open. Fiddling with the switch would bring it on. Closing the door always turned if off. Just ON was a problem. Again, the courtesy light had control switch had:

Always ON Gated by door switch(es) - wired OR Always OFF

I would work on my vampire loads, before giving in to the appetites of the car.

My current car has vampire loads of less than 20mA. The meter I used, can't give readings better than that (that's the clamp-on ammeter reading). I would need leads with banana plugs on the end, to be making a current measurement without disturbing the electrical system.

My charger has no problem handling the 20mA vampire load. And, it automatically cycles when the Vbatt drops low enough. The only thing my charger doesn't do, is there is no temperature compensation. The voltages it uses, correspond to operation at 25C. If the air temperature was somewhat different, the battery would be under-charged or over-charged accordingly. There is a least one commercial charger, where the "temperature measurement lead" is a separate item, and you can place the lead where ever seems appropriate.

If you change battery chemistries, then it's possible some charger details will change too.

Paul

[jon]

I used an old 19v x 4A laptop power supply with a couple of 55W headlight bulbs in series to charge my flat battery. It started at 2.5 amps and dropped to 1.8 amps over a couple of days, giving a full charge.

[newshound]

On #1, I have lead acid batteries for lights and electric fencers in a stable block. These are usually ex-car ones, so often a bit tired, and they do sometimes get run right down. I often need to start them off using an ancient transformer type with a moving iron ammeter. If you don't have one of these, look out in car boot sales.

I also have an older version of one of these

Mine has a transformer and I think this is likely to have one too. Occasionally handy as it will run off a small genny.

[Harry Bloomfield, Esq.]

Brian Reay explained on 21/07/2020 :

The Lidl one is suggested to be better built than the Aldi version.

Which is fine, there is no need for current measurement, as with old fashioned dumb chargers - the charger takes care of current.

I didn't know that, until years after I bought it, I watched the video. I thought the flashing display simply meant it was unable to attempt to charge (I never read instructions).

The older smart chargers, the only way to get them to charge was to initially parallel the flat battery with a charged one.

I agree it is just plain daft, not to fix the discharge/ the boot light left on. It is a great way to quickly wreck a battery and so easy to fix. Modern cars turn their interior lights off, if the car is left undisturbed for a while. Leave my car's door open for 20 minutes without otherwise disturbing the car and the interior lights go off. I going a stage further - I ordered myself a fancy micro-based battery analyser last week, due for delivery today. I have quite a few batteries around the place and battery capabilities are notoriously difficult to assess effectively.

[Harry Bloomfield, Esq.]

Paul presented the following explanation :

With my car it is easy to test discharge whilst parked. There is a copper link/busbar linking end of battery lead to the under bonnet fuse panel, which has all the very large main fuses in it. I just connect mA meter between battery +ve post and fuse end of bus bar and remove the link. The measurement will not include any discharge via starter motor, or alternator, but those are much less likely self discharge culprits.

[Scott]

What about a time delay switch? Could you get one that cuts off after (say) 2 minutes?

With respect, I think the original question is the wrong question :-) Surely the correct question is how to find the best solution to the problem.

[Brian Gaff (Sofa)]

To build one is not that hard though a beefy bridge rectifier and a transformer salvaged from some high power low voltage device, probably find something at a boot sale. Brian

[Dave Plowman (News)]

Most modern chargers won't work with a totally flat battery. They use battery volts to pull in the reverse connection protection relay. So you may need to use a jump battery to get it started. Other than that, most will limit the current demanded to what they can deliver.

[Theo]

Having taken the thing apart, it turns out the bulb holder is actually part of the switch mechanism, and one end of the bulb moves up or down when you flip the switch.

So mechanically it's a bit tricky, but I think I can do something with:

- intentionally blow the bulb, so I have the glass envelope to use as a substrate

- fit some white LEDs and a resistor across it. The LEDs are chosen such that the forward voltage is set to match say the '25% battery' voltage (looks like 12.0V). When the battery voltage falls below that level the LED current collapses and the load naturally falls away.

It would need some tweaking to get the current curve right, and it's possible a more sensitve cutoff circuit would be needed. Worth a try though.

Well indeed, but 'I wouldn't start from the bottom of a hole' is not an answer to a question about the best set of grappling irons.

Theo

[Dave Liquorice]

Apart from that through the resistor across the LED and modern white LEDS take naff all current. You can get LED versions of pretty well any automotive bulb that is out there apart from some headlight bulbs. You'd not want ones that are compatible with some cars "bulb failure" systems as they have resistors to ensure they draw enough current to supress the "bulb failure" warning.

The correct solution is to work out what stops the boot from not closing properly and fix that. Bit of lubrication? Small adjustment to the pin that the lock striker engages with? Still if trying to develop a complicated workaround, ignoring the root cause, keeps some one happy...

[Theo]

In this case the circuit is carefully designed to be sensitive to voltage. A regular automotive indicator LED bulb is a 3.2V white LED and a series resistor which drops about 10V. The resistor limits the current so the forward voltage of the LED is immaterial. In my setup the current is very much dependent on the forward voltage, which is the point.

I'm not sure I'd be able to find LEDs when put in series with quite the right voltage/current curve though. It might need a more conventional cutoff circuit.

In this particular case, a load of cardboard fouling the lid. Root cause analysis doesn't help when the root cause is the idiot user. Upgrades to the idiot may be unsuccessful.

Theo

[Scott]

I cannot imagine any of this is good for the battery. It seems to me a destructive way of solving a minor issue.

[williamwright]

A voltage reading on a battery that's under charge tells you very little about the battery. It tells you a bit about the charger, that's all.

Bill

[The Other Mike]

Angle grinder to remove the boot lid, duct tape to insulate the bare cables.

Even buying both new should work out cheaper than a new battery charger.

Or fix / replace the switch

[williamwright]

Fit a little intermittent bleeper across the boot light. Easy and cheap.

Bill