I have a van with two batteries.
If I take one battery out and put a completely dead battery in instead (not even enough juice to run an electric pump) I can start it with the good battery and then the alternator will charge both.
How long would i have to drive around for before the dead battery was fully charged again?
I don't have a multimeter to just measure it and check. Well, I do but it's broken, need to get a new one.
Thanks
I don’t have a multimeter to just measure it and check
If this is something you care about, and it appears that you do, just buy another. It will also help you assess whether the battery is just dead or no, which it sounds like it might be.
Thanks. There's one on order, just been on the google since writing that and ordered one but hoping to have the battery charged before it arrives.
Not dead, long and not very interesting story, but definitely takes and holds charge.
I'll be driving about 30 miles on Wednesday in three short trips, wondering if this will be enough? Probably another 20 miles on Friday but would like it charged then.
Short trips won't do it any good. Will probably take more out starting the vehicle than it will put back in.
Three things
If the battery's as low as you say it's almost certainly damaged and will be operating at a lower capacity going forward.
And alternators are terrible as charge controllers -if there was ever a way to guarantee point 1 it would be to use an alternator to change from dead.
In an ideal world you want to be using a smart charger to phase the level of charge to the battery over a 12-18 hour period. In an even better world -if the smart charger had a recovery feature it would be better.
You may struggle to get a basic charger to register a battery as dead as you say as it will not see a very low battery
At least 5 hours driving to get it ~80% charged (assuming the battery is still ok). Ordinary lead-acid batteries won’t accept more than ~20% of their capacity as a charge current (other than briefly). Above about ~70% charge this tails off further.
What trail_rat said. The answer to your question (assuming the battery is not actually dead and will take a charge) is it will depend how big the battery is, how much output the alternator can deliver, and what equipment is turned on in the van when charging.
Crudely, a 60 Ah battery will charge in half the time of a 120 Ah battery.
The alternator on a VW caddy is probably rated at something like 90 A, a big van might be about double that. In either case, it will not put out anything like its full capacity at tick over. If you have the lights, radio, fan, wipers etc on then at tickover they'll be drawing almost all the power the alternator produces leaving none to charge the battery. Obviously you want any van equipment like fridges, internal lights, technology turned off to help the charging.
Even with a big van and relatively small battery I doubt you'll get from "0" to "100%" in 3 short trips totalling 30 miles.
Just be careful that attaching a dead battery across the good one, doesn't draw such a big current from the good one that it kills it too - does it have an isolator switch? You'd be able to get a charged from Halfords etc easily and do it properly (even a pretty poor quality charger is probably a better way to treat a dead battery than blasting it from the alternator); although by the time you do that you may consider just getting a new battery... they really don't like being completely flat, so I predict if it has not failed yet it will soon as a result.
Yes, watch your good battery as, once the poorly one is connected to the same system, power will flow from the good battery to the bad one thus bringing your good battery down.
Ideally you should install an automatic charge relay (ACR) which will keep the batteries isolated from each other (to stop ^^ happening) until the main battery is charged then it connects them so they both get charged by the alternator.
On your Mk6 Transit the two batteries are just wired in parallel. The Mk7 has a split charge relay. I would suggest a bench charger, they're not expensive, but the batteries are a pain to get out if you can't charge in-situ.
If the dead battery is out of the vehicle I would go and buy a charger from Halfords, etc, and just leave it charging from the mains in well ventilated place. Much more likely to result in charged battery and less risk to the alternator and the other battery.
Thanks all.
Yes, it's the battery from the Mk6 going into the Mk7.
Hadn't considered the flat battery drawing current from the good one, will leave disconected when not driving to prevent this.
Battery charger has died along with the multimeter, hence going down this route. I should just cough up for a new one, will need one at some point but not sure when I can get to town to get one. And yes, charging in situ isn't possible, we don't have a driveway and park some distance from the house.
"Hadn’t considered the flat battery drawing current from the good one, will leave disconected when not driving to prevent this."
youd be better just leaving it disconnected till you can get to town for a charger, your opening a whole can of worms potential vehicle fires, melted wiring blown fuses and cooked alternators - any or all of the above tbh.
