What happens if...

[rlphoto]

what happens if I plug a 8ah or a 16ah battery strait into an automotive cigarette lighter to try and charge it?

[Dale M.]

Would probably work...... Probable not best idea....

Two things may be issue... Current in rush (amperage) to battery from car system may cause accessory socket fuse to blow.. Voltage differential in "charge capacity" may cause battery to charge to fast and cause large gas build up in case and cause excessive pressure and case to split and then you have toxic clean up issues...

A simple 12 volt bulb in series with charge circuit as a current limiter can be a great safety valve to prevent above scenarios... Also it may act as a charge indicator... As bulb gets dimmer it indicates battery is nearing full charge...

Dale

[brian_bp]

This is done routinely. For instance, all those portable power packs (which may include things like an inverter, a 12V power socket, booster cables, air compressor, work lamp, etc) may be charged with the included crude AC charger "brick", or by plugging directly into the the car power socket; mine came with a male-to-male cord to connect the car socket directly to the power pack socket for exactly this purpose. I've charged mine both ways (as well as by solar panel). They normally use AGM batteries.

On the other hand, the initial charge rate may be higher than ideal, as Dale mentioned. The ideal charge rate is best considered in terms of current, but battery manufacturers will provide charging voltage specs; for instance, Optima says (for all sizes of their deep-cycle and dual-purpose AGMs):

Rapid Recharge:
Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below 1 amp.

I guess that appropriate questions might be:

Is there any chance that the voltage at the battery to be charged - after loss due to current flow in the wiring - is more than 15.6V?
Will the battery be monitored to ensure that it isn't getting too hot?

I think I would be more concerned if the battery is a gel or flooded cell type than I would be with an AGM.

[Alphacarina]

A simple 12 volt bulb in series with charge circuit as a current limiter can be a great safety valve to prevent above scenarios... Also it may act as a charge indicator... As bulb gets dimmer it indicates battery is nearing full charge...

Excellent suggestion in this case - Using as large a wattage bulb as you can find will make the charge time less than if you use a smaller bulb. Something like an 1156 backup light bulb would be a good choice

Don

[BPFox]

what happens if I plug a 8ah or a 16ah battery strait into an automotive cigarette lighter to try and charge it?

How do you make the battery small enough to fit in such a small hole?

[wannabefree]

The type of battery you are talking about is most likely a sealed lead acid type (SLA). SLAs are designed to vent if abused, giving off hydrogen gas. If they don't vent, they explode.

The kind of charging you are talking about is "abuse" as defined by every battery manufacturer I know of. The cheap battery booster mentioned above probably has some current limiting, if even just a resistor, built into the charging cable or in the box. If not, well, thanks, China.

The light bulb idea is a good one. If you go that route, a safe charging rate is C/10, where C is the battery's capacity. So a 10AH battery can be safely charged at 1A. So find a bulb that draws 1/10 or less the current of your batteries. The light bulb is also a handy charge rate indicator -- it gets dimmer as the battery charge gets closer to full. Just make sure the bulb is in series with the battery, not in parallel.

If you want to fast charge a battery the key is thermal monitoring. While the Optima web site may state that the charge rate is unlimited, they only make that claim with the thermal limit caveat. Fast chargers do monitor the temperature of the battery they are charging and adjust the charge rate accordingly. And Optimas are not your everyday batteries.

A notch better than the light bulb is to get a cheap solar charge controller and put it between your battery and your vehicle's electrical system. I have seen them for under $20.

As far as the worst that can happen (you didn't ask, but I can't resist) is the battery exploding. Not good if it's sitting in the passenger seat while you drive down the highway. At best you will severely limit the life of your batteries.

I guess I should say this is all my opinion and others may differ. If you would like a second opinion, try www.batteryuniversity.com. I'm an engineer, but this guy builds chargers for a living.

Good luck and play safe,
Sherman

[brian_bp]

The type of battery you are talking about is most likely a sealed lead acid type (SLA). SLAs are designed to vent if abused, giving off hydrogen gas. If they don't vent, they explode.

The linked source mixes descriptions and terminology for sealed batteries in a confusing way. The more common terminology of Valve-Regulated Lead Acid (VRLA) includes both designs with gelled electrolyte ("gel cells") and those with electrolyte absorbed in a glass mat (AGM). With any luck, the battery is of the newer AGM type. In any case, yes they will vent if sufficiently provoked.

The kind of charging you are talking about is "abuse" as defined by every battery manufacturer I know of. The cheap battery booster mentioned above probably has some current limiting, if even just a resistor, built into the charging cable or in the box. If not, well, thanks, China.

I doubt that there is any current regulation in the booster. If it were a resistor, it would need to dissipate quite a bit of heat at the maximum charge rate, and there is neither a part which gets hot nor an apparent heat sink. The built-in charge controller simply switches off at a set voltage (e.g. 14.2 V) and on again at a much lower voltage (e.g. 13 V); it does not perform anything like pulse-width modulation, so the current is unregulated when it is flowing at all.

If you want to fast charge a battery the key is thermal monitoring. While the Optima web site may state that the charge rate is unlimited, they only make that claim with the thermal limit caveat. Fast chargers do monitor the temperature of the battery they are charging and adjust the charge rate accordingly. And Optimas are not your everyday batteries.

While Optimas are not the most common battery, the AGM design (used for the Optima) is very common - I think nearly universal - among these portable battery packs and small UPS units... rlphoto's intended battery may well be an AGM.

I'm not suggesting that this battery be deliberately abused, only that the direct connection to the lighter socket (with small-gauge wire on both sides of that connection providing substantial resistance) may not be so unreasonable, and appears to be the expected use of some commercial products.

A notch better than the light bulb is to get a cheap solar charge controller and put it between your battery and your vehicle's electrical system. I have seen them for under $20.

I have a cheap solar charge controller - Canadian Tire item 11-1890 or SunForce #60012 - although it is rated for up to 7 amps, it does no current control (just the on/off switch)... it is assumed that you will use this device only with a power source which is not capable of delivering excessive current.

I expect that a solar charge controller with a PWM current-regulation mode will cost substantially more than $20, but would do a nice job in this situation.