do i really need to disconnect battery to charge?

[fast but dim]

As stated, my 120 has 2 batteries, I'm giving them a pre-emptive top up today.
I've disconnected the earth on the one i am charging, but do i need to? could i leave the leads on and charge both at the same time without causing harm?
Its a 2/4 amp smart charger btw.

 

[TonyP]

Depends on if they are flat or not. If you only topping them up I would leave them connected.

When my alternator stopped working after a stint down in Salisbury Plain I charged my batteries in situe every night without disconnecting anything, worked well enough for me.

 

[uHu]

You can normally charge without disconnecting. Particularly with such a small charger (would take you days to charge flat batteries).

Reasons for disconnecting before charging:
If you charge a battery which has an "open" cell, the voltage could go very high, depending on the charger. If you have a big, simple charger with a boost function, the voltage could in such a case go up to 20 volts, which could be damaging for the car electronics. But as long as the battery works well, and you use a normal charger, you should be safe. On a good standard battery, the voltage goes up to 14.4 volts, or just a bit over. Some types (silver calcium or agm) have a slightly higher voltage.

PS:
An open cell means that there is no chemical reaction, and the cell doesn't give any load to a charger. Therefore the voltage over that cell, and over the whole battery, would go as high as the charger can supply without any current flowing.
PS2: (While we are at it)
Another condition is a shorted cell, where the voltage of the battery will be two volts lower than standard. When charging, the current would stay high without getting up to the nominal voltage.
PS3:
Open cells can sometimes be fixed...

 

[yogi]

In theory as stated above not really a problem, depends on WHY you are charging them though. If you are losing charge due to a short etc then this will continue to happen so if you are using a crappy charger it will increase the charging time to............ well potentially infinity.
Its prob just easier to disconnect them and just do nthe job right!!

 

[Iwan]

With a dual battery setup, it is recommended that you swap the batteries over from time to time. The lead battery that has the alternator hookup can take more of the charge power than the 2nd. During the winter on my cruiser the 2nd battery was reading low so after the swap over it regained a fuller charge.

I would charge the batteries disconnected as it reflects to the description above and rules out any electrical damage the may occur. Hope it helps!

 

[Shayne]

I have a dual 12v battery set up on my boat and after being left to run flat , i got the boat started via jump leads but found the battery alarm often sounded despite the boat being in use all day every day . I assumed i'd killed one of the batteries and it needed replaced . While cleaning the connections just to be sure i decided to swap them over . Both batteries now work fine .

Also i'm inclined to think i have a very minor short somewhere that leaks battery power , several investigations by several people have failed to find a fault , boats only 2 years old ? . Anyway i have been wondering if adding a solar trickle charger might just counteract the leak so batteries remain charged even if the boats laid up for a while .

Don't know much about solar trickle chargers nor whether they can remain permanently attached but it might be worth looking into ?

 

[Graham]

With a dual battery setup, it is recommended that you swap the batteries over from time to time. The lead battery that has the alternator hookup can take more of the charge power than the 2nd. During the winter on my cruiser the 2nd battery was reading low so after the swap over it regained a fuller charge.

I would charge the batteries disconnected as it reflects to the description above and rules out any electrical damage the may occur. Hope it helps!

.
.
Hi iwan,

That's an interesting take on the duel battery set up's.

You probably know that all EU 120 diesel D4-d series Land Cruisers, come out the factory with two standard front end mounted batteries. (There may be other Land Cruiser models, but as mine is a 120 diesel D4-d series, I just make point)

Would there really be any benefit ?
My 120 series batteries are interconnected with some serious HD cable, so I can't understand how one battery can give or take any more power than it's partner.

Also the batteries are left and right handed.


Gra.

 

[Crispin]

I'm with Graham on this. The connection between the two batteries is large (not actually measured it) but the resistance on it would be of no consequence with charging current. The negative path is via the chassis so again. With a max charge of 120A from alternator you might find a small drop but as the battery reaches full charge the current will drop off as will the voltage drop.

I would think if you have a significant difference between them I would start looking at the connections for a problem.

As for the original question, charge them separately, disconnected from vehicle would be best. Caution though when reconnecting the just-charged battery back to the vehicle - the not-yet charged battery will slurp a lot of current from the newly charged one. You'll be treated to a small spark as it connects. (Small chance of there being hydrogen from the charging)

 

[Roger Fairclough]

I'm with Graham on this. The connection between the two batteries is large (not actually measured it) but the resistance on it would be of no consequence with charging current. The negative path is via the chassis so again. With a max charge of 120A from alternator you might find a small drop but as the battery reaches full charge the current will drop off as will the voltage drop.

I would think if you have a significant difference between them I would start looking at the connections for a problem.

As for the original question, charge them separately, disconnected from vehicle would be best. Caution though when reconnecting the just-charged battery back to the vehicle - the not-yet charged battery will slurp a lot of current from the newly charged one. You'll be treated to a small spark as it connects. (Small chance of there being hydrogen from the charging)

Ditto the first part.

I don't disconnect my batteries when charging. I charge both together---basically a top up every few months---at around 8 amps. Charger cuts off after maybe 10 minutes. It's not a good idea to promote any spark at a battery post. A colleague did it once and the explosion ripped the battery apart. His clothing was drenched in acid but by some miracle he wasn't. The ringing in his ears was a reminder of how lucky he was.

Roger

 

[Crispin]

Not sure if it matters for auto application but I read that charging a battery with "not enough" current is also not advised. I think it is more for batteries which are cyclic application. It's to do with clearing the plates on the lower half of the battery. Because of this, I don't charge them in parallel (halved current to each battery)
The more I read on the topic the more conflicting opinion and "facts" there were.

On the spark / explosion side, the owner of an electronics shop I used to frequent as a kid showed me photos of a friend of his who worked on the mines. He dropped a spanner on a bank of batteries in one of the tunnels. The explosion and acid did him some serious injuries. Pics were of him a good few years later and still lots of scares. Always cautious with Pb and charging...

 

[Roger Fairclough]

After the blast, I rushed across to him---I was on the other side of the workshop. He was standing there with a dazed look on his face. The battery, or what was left of it, the top was completely missing, was less than 3 ft away. How the acid missed him was a complete mystery.

Roger