unusual way to charge a car battery

[Nigel Goodwin]

I don't think I've commented before?, but the idea is perfectly valid - it's absolutely horrible, and a nasty idea, but it will work prefectly well.

However, I don't suggest it's something you should build, if you're building a battery charger you may as well do so properly - but I can see it might be useful for throwing together if you're stuck with a flat battery one night!.

 

[Grant Fleming]

Can't you picture it?

There isn't a schematic but it's so simple it doesn't need one, the description shloud be good enough. Alright for nubesake I'll draw one so everyone's clear on what he means.

Look, notice how the bulb is in series with the bridge so it limits the current and the voltage across it is lower than 230V.

However, I do agree this isn't a good way of doing this, it dangerious and wasteful, you're better of using a transformer, rectifier and LM317 constant current source.

Might be better picturing it!
This schematic has bulb in different place- (after all the discussion on the d.c voltage that would appear across the bulb).
Cheers,
Grant

 

[Nigel Goodwin]

Might be better picturing it!
This schematic has bulb in different place- (after all the discussion on the d.c voltage that would appear across the bulb).

Makes no difference, essentially all three parts are in series with each other, it doesn't matter which goes where - and a bulb will work exactly the same on fullwave rectified DC as it would on AC.

 

[Grant Fleming]

Makes no difference, essentially all three parts are in series with each other, it doesn't matter which goes where - and a bulb will work exactly the same on fullwave rectified DC as it would on AC.

I know it makes no difference! If you read my previous post in conjuntion with the original post and thread discussion at great length over the d.c being on the bulb, then you will see the schematic doesn't describe it.
Cheers,
Grant

 

[Hero999]

I'm just illustrating Nigel Goodwin's point; it doesn't matter, this circuit will have DC on the bulb and it won't damage it.

 

[oldtimer]

It would work of course for a while and then when the battery is ruined through over voltage at the plates causing holes which causes debris in the bottom which causes shorts. Put a crowbar in circuit and it will improve it but don't put it near my batteries.

 

[Hero999]

No, it won't overvoltage since it won't charge to more than 14V providing the current is limited to a safe value, in theory you could connect it to a 1MV source via a super high-power, high-voltage 2Mohm resistor and it'll never over voltage since the current is limited to the safe value of 500mA.

 

[oldtimer]

The army used them when there was a large number of batteries to charge with a large rheostat or sometimes three or four. Better to stick to voltage reduction through mutual inductance ie a transformer then rectify it then through a regulator circuit. You could use a SCR perhaps and pulse it then store the output in a capacity bank,that would be safer.

 

[Hero999]

You could even use a transformer with an oversized secondary and large leakage inductance to limit the current, then you wouldn't need a power wasting resistor.

 

[oldtimer]

yes you could.

 

[xrunner123]

No, it won't overvoltage since it won't charge to more than 14V providing the current is limited to a safe value, in theory you could connect it to a 1MV source via a super high-power, high-voltage 2Mohm resistor and it'll never over voltage since the current is limited to the safe value of 500mA.

Why wouldn't the battery see more than 14v? The battery will zener at 11-13V (depending on depth of discharge) when charging but what happens when it's charged? The terminals slowly start to rise towards the rectified voltage, outgasing will occur, and you'll damage the battery if not explode it.

Actually when the battery is accepting a change (full charge, or partial) you don't even need a current limiting device- using the batteries internal resistance is perfectly fine to limit current, but when the battery is at near full change the voltage must be reduced to the approriate float voltage level.

 

[Hero999]

That may be the case in a car when the alternator voltage is never any higher than 14V but this isn't the case here. The battery won't be damaged since the current is limited to <400mA by the bulb, which is a safe for a trickle charge.

 

[xrunner123]

That may be the case in a car when the alternator voltage is never any higher than 14V but this isn't the case here. The battery won't be damaged since the current is limited to <400mA by the bulb, which is a safe for a trickle charge.

So there is a rectified 230ac with a 100W bulb in series connected to a 12V battery external to a car? Correct? Why is it that the voltage never increases past 14v again? Can you calculate this? Maybe I'm missing something here? Don't forget a battery is only a current fed device when it is accepting a charge. Have you actually tried charging on this way?

 

[Hero999]

I've never tried to charge a 12V battery in this manner, I've charged one from constant current sources with open circuit voltages far greater than 14V and the voltage doesn't increase much beyond 14 once the battery is fully charged.

 

[xrunner123]

I've never tried to charge a 12V battery in this manner, I've charged one from constant current sources with open circuit voltages far greater than 14V and the voltage doesn't increase much beyond 14 once the battery is fully charged.

So what holds the voltage at 14v?

I assure you that you will outgas and damage the battery with a float level above 15V @25C. You cannot get around this without controlling the battery voltage. There's a reason why battery manufacturers specify a maximum float level.

 

[Nigel Goodwin]

So what holds the voltage at 14v?

The battery does, it's only trickle charging at very low current, not anywhere near enough to overcharge and cause damage. The disadvantage is how long it takes to charge!.

 

[Hero999]

I assure you that you will outgas and damage the battery with a float level above 15V @25C.

Was a 500mA LM317 constant current source run from a 15V transformer, the open circuit voltage was about 18V, the battery only charged to 13.8V and bearly got warm.

You cannot get around this without controlling the battery voltage.

Or current.

There's a reason why battery manufacturers specify a maximum float level.

You're talking about constant voltage float charging, we're talking about constant current trickle charging, the two are totally different.

 

[xrunner123]

Actually you don't have to control current. The battery will accept any current limited by the internal resistance. And I am talking about a CV type charge and almost all manufacturers will specify a float voltage level. The first charging stage is constant current, then constant voltage- even a car uses this type of charging and it’s the most commonly method used.

CC (Constant current) charging can be very dangerous to the battery due to the fact of applying greater than specified battery voltage which leads to outgassing. You guys are talking about 500mA as a safe level to trickle charge- I highly doubt it is. Most lead acid batteries can be left in a constant current rate of 0.001-0.002 x Crate for long periods of time. So for a 20Ah battery this leaves a 20mA CC charge. I’ve designed a few chargers (100W-2Kw) in my life and never used this method because it’s frowned upon by battery manufactures. The most reliable method is CV- constant current then constant voltage.

You can get away with using a CV with an overcharge cycle. This improves charging time. Charge with constant current until the current tapers off to about 0.02C then increase the battery voltage to 15V or so (depending on temp and charge rate)- charge at this voltage until the charge rate is returns back to 0.02C, then return to a float voltage. This is a little more tricky since during the overcharge portion is where the outgassing can/will occur so care must be taken not to overdo it.

 

[Nigel Goodwin]

CC (Constant current) charging can be very dangerous to the battery due to the fact of applying greater than specified battery voltage which leads to outgassing. You guys are talking about 500mA as a safe level to trickle charge- I highly doubt it is.

Doubt it all you like!, but commercial trickle chargers have been sold since well before WW II, and are still sold today - and don't appear to cause any damage to the car batteries they are used on!. From what I can remember, they usually charge about 1A?.

 

[Death By Bass]

if the current is limited to 500mA, whats stopping that from being 500mA at 200+volts DC?

I've killed a battery by accidently charging it to 18volts, with a 2.5amp current limited powersupply.

whats stopping 500mA from doing the exact same thing?