Connecting main & leisure batteries in parallel

Nick · Post by **Nick** » Mon Sep 04, 2006 12:26 pm

My Bongo has the dual factory-fitted main batteries. One of these is knackered and it starts ok on the other one. I'm about to buy a leisure battery rather than an new main battery. I propose to disconnect the main battery when camped and reconnect while driving (by manually detaching/attaching the earth lead from the main battery). I hope this will work as a simple and cheap solution which will avoid having to buy and wire up a split charge relay. During driving, with both batteries connected in parallel, the leisure battery will need recharging but the other won't (after camping up for a few days). My questions are:

1. Will the alternator charge them both correctly, or will it over-charge the main battery (which won't need charging much - only the charge that it used starting the engine at the end of the camping)?

2. Will use of the starter while both are connected in parallel damage the leisure battery, as it is not designed for use with starter?

Veg_Ian · Post by **Veg_Ian** » Mon Sep 04, 2006 12:52 pm

Your solution sounds a bit tedious, connecting and reconnecting batteries and if you forget one morning, yes you could damage a leisure battery by trying to start the engine. Fitting a split charge relay isn't that expensive or tedious but if you really want a simple life, I would replace your duff battery with a car battery. Probably cheaper and offers a safer compromise.

grumpo · Post by **grumpo** » Mon Sep 04, 2006 1:40 pm

If it were me, given the same circumstances, I would go for the Veg_Ian
suggestion, however I doubt that you would ruin a leisure battery that
easily.

Traction batteries are designed with as many as possible very thin plates
which can supply the necessary cranking current needed when starting the
engine. Because of this, traction battery plates will start to buckle if their
capacity is reduced to below about 70 percent, the most common cause
of battery failure.

Leisure batteries on the other hand are designed with much fewer thick
plates and can withstand the buckling effect easily down to about 40
percent capacity. It's the number and size of the plates which determine
the maximum current available from the battery. As both types have the
same voltage characteristics, it would make little difference to their
respective charging rates.

My concern would be in connecting a completely discharged leisure battery
to a fully charged traction battery. If this were done the traction battery
would instantly discharge in to the leisure battery with a fairly high current
and if you did not start the engine immediately you might drain the traction
battery enough to cause you starting problems, bearing in mind you may
have to use the glow plugs.

As you already have dual traction batteries fitted, I would consider using
the existing installation a much safer option even though you may have to
keep your eye on the battery voltage and make sure you don't discharge
them much below 13 volts.

moonshine · Post by **moonshine** » Mon Sep 04, 2006 10:53 pm

There does seem to be a popular misconception that starting an engine off a leisure battery will in some way damage it. It is a common practice on most boats. My boat has an ordinary engine-starting battery connected to the starboard engine, which is just used to start that engine and nothing else. On the port engine I have two leisure batteries in parallel. These are used to start that engine and provide all the domestic electrics, pumps, navigation lights etc. A system of changeover switches enables me to start the port engine off the starboard battery if I have flattened the leisure batteries, but this has never been needed, even when I inadvertantly left my (compressor) fridge switched on all week. Mind you, the total leisure battery capacity is 220ah.

I totally agree with Grumpo. I would not be happy to connect a discharged leisure battery directly to a starting battery, except through a split-charge relay as one will immediately start to discharge into the other. There will also be a huge spark when you try to put the terminal on.

chiefster1976 · Post by **chiefster1976** » Sat Oct 14, 2006 10:34 pm

I have 2 batteries. The one on the right hand side has a link wire joining to the left side battery WIRING.like a T piece effect. Not directly coupled to the left side battery terminals.
I have only ever seen starter batteries directly linked to the terminals..Why is this???
As the batteries look identical and i THINK with the wires could be linked to one another and there is no lesuire extras inside the vehicle i presuming that the second battery is indeed another starter battery????

If this is the case would i just have to pay out for a lesuire battery and split charger to use in the vehicle.....Total weight of 3 batteries would be on the vehicle????
Thanks.....Gavin

moonshine · Post by **moonshine** » Sat Oct 14, 2006 11:33 pm

There is probably no need in this country to have two engine starting batteries as you appear to have, these are really only for very cold climates. The best solution would be to replace the one on the nearside with a leisure battery and wire in a split charging system. This is quite simple to do, and involves disconnecting the heavy positive cable currently connecting the two existing batteries, and connecting the new leisure battery to the offside one through a split charge relay. These are available from auto electricians, caravan centres, or Halfords. Fitting instructions are generally on the packet.

chiefster1976 · Post by **chiefster1976** » Sun Oct 15, 2006 2:44 pm

moonshine wrote:There is probably no need in this country to have two engine starting batteries as you appear to have, these are really only for very cold climates. The best solution would be to replace the one on the nearside with a leisure battery and wire in a split charging system. This is quite simple to do, and involves disconnecting the heavy positive cable currently connecting the two existing batteries, and connecting the new leisure battery to the offside one through a split charge relay. These are available from auto electricians, caravan centres, or Halfords. Fitting instructions are generally on the packet.

Hi thanks for the info....So basically i CAN use the existing cable connecting the two batteries together, just cut the cable and insert a split charger. Is this correct? Sorry not very electrikery person!!
How much would i exspect to pay for one?? WE have a maplins near me, are they the best option?
Thanks matey..Gavin..

moonshine · Post by **moonshine** » Sun Oct 15, 2006 5:26 pm

Hi Gavin,

Yes, in theory that is exactly what you do, but in practice you do not need such a heavy cable just for charging the batteries. The cable linking them at present would be starter cable capable of carrying about 400amps, which would be impossible to connect to the split charge relay as it is much too large. What you need is some 70amp cable (or similar) to connect up your split-charge system.

Maplins would almost certainly be able to supply the bits, but if not, Halfords, a caravan spares shop, or a towing shop would have them. If you are doing it yourself, the parts should cost no more than about £40, I would think, including cables.

Regards,
Colin

chiefster1976 · Post by **chiefster1976** » Wed Oct 18, 2006 10:13 pm

moonshine wrote:Hi Gavin,

Yes, in theory that is exactly what you do, but in practice you do not need such a heavy cable just for charging the batteries. The cable linking them at present would be starter cable capable of carrying about 400amps, which would be impossible to connect to the split charge relay as it is much too large. What you need is some 70amp cable (or similar) to connect up your split-charge system.

Maplins would almost certainly be able to supply the bits, but if not, Halfords, a caravan spares shop, or a towing shop would have them. If you are doing it yourself, the parts should cost no more than about £40, I would think, including cables.

Regards,
Colin

Thanks moonshine for the info....As i am eventually converting my bongo with full conversion i am aquiring most parts from an ex employer....I.E. the PMS unit..AS i never had the full training with this i was going to let them fit the pms system...Any idea what a good price would be for the fitment of a lesuire battery + cables+ split charger+and pms unit ( pms unit will work the electric pump, aux lights,cooker elec ignition, 3 way fridge....it will have a van/aux switch.
Not really sure on what others options i would need at present...

moonshine · Post by **moonshine** » Wed Oct 18, 2006 10:36 pm

Haven't really got a clue about what anyone would charge to do this job Gavin, as it's the sort of thing I would do myself if I intended to go that far with my Bongo. I have done it on a boat, but that was such a long time ago that I can't remember what the price of all the bits was. I have owned three boats now, and have completely rewired all of them adding all sorts of modifications such as this. No such thing as a handbook or workshop manual for a boat, as every one is different and you have no option but to work it all out yourself. At least with the Bongo we can take it to an auto-electrician.

I am sure however, someone must have an idea of the cost of what you intend.

Colin