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[Shackeng]

I have one of the Halfords 12v 34AH jump start boxes, and have found it very useful, even though I've only needed it once myself. It has a plug-in type charger, 16w 13.5v 500Ma, and connects to the box with a mini jack plug. As the unit rarely gets used, I have to remember to charge it regularly. Is there any good reason not to leave it permanently on charge ?

[Kevin Wood]

Depends on the charger. From the description, it sounds pretty crude, so I would hesitate.

Best thing to do is to leave it on charge for a few days and then measure the voltage across the jump start terminals with a multimeter while it's on charge.

A safe level of float charge for these type of batteries is about 13.6-13.8 volts. If it's sitting higher than that, constant charging will probably not do the battery much good.

[Shackeng]

Thanks Kevin, I checked the voltage at 15.25v just now with the charger connected and the battery fully charged (13.13v).

PS, I have a golf trolley 3 stage charger which might be more suitable, if I can bodge the lead to fit the jack.

[Kevin Wood]

That's a bit high. It will cause the battery to gas, which is the last thing you want in a sealed lead acid battery.

The second last thing you want is to leave it to discharge significantly in storage, however.

Might be worth investing in a smart charger for it. Aldi frequently have clones of the CTEK units for around £10-15.

[Shackeng]

Thanks Kevin.

[Mr Skrunts]

Would it work to fit a mains timer, charge for 1 day a week.

[Bigron]

This is not a cost-effective solution, but I have a 13.8 volt high current stabilised power supply left over from a previous hobby which I am happy to leave trickle-charging my battery forever because I have the fairly commom 100mA quiescent current drain on my Omega.
As Kevin rightly says, an voltage significantly higher than 13.8 volts for an extended period will "gas-off" the water content of the battery if unsealed or give it a bellyache if sealed, also resulting in sulphation due to a more concentrated electrolyte. Don't do it!

Ron.

[ozzycat]

as far as i know which is not alot  the halfords jump chargers are made by the same crowd that make the blackspur ones {the bright yellow ones} and they are va load of horse manure i may be wrong but bif it works so what

[Bigron]

A caveat regarding some (all?) smart chargers: they usually pulse-charge the battery, i.e. on-off-on-pff etc., with the battery's voltage being monitored in the "off" period to determine state of charge and switch back to trickle when fully charged. That's great, but unfortunately it's unwise to use this type of charger whilst the battery is still connected to the car because the ECU does not like the pulsed nature of the charging current and may die.
They work well if you disconnect the battery completely, however.

Ron.