Batteries

[Coops]

Simple question if you know the answer, which I don't. Wanting to double up my battery capacity by adding another 110 a/h battery in parallel for use in conjunction with a solar panel, but don't know how the two batteries will react if they are unequally charged at the time of linking up. Will they equalise like water in two tanks ?

Grateful for any replies.

 

[ProfJohnL]

Hello Coops,

The simple answer is yes, they will try to equlise, but there is a potential serious problem. If the condition of the batteries is different, then it is possible or even probable that the charge differental between teh batteries will cause large (dangerous) currents to flow between the batteries. For safety you should only connet two similar batteries of the same type and age.

 

[Woody3]

There is a simple and safe solution. Connect say the neagtive terminals together and then link the positives using a car headlamp or domestic 100W incandescent bulb. If the light glows leave it for an hour or so; if they don't glow then you can be farily certain that the batteries are close enough in charge to be connected together.

Another option is to connect one battery charger to both batteries at the same time but each battery via its own light bulb and leave it on like that for a a few hours (if the batteries are in fairly charged state) or a few days (if discharged.) The batteries will then be roughly equally charged and can be safely parallel connected without irsk.

 

[Martin24]

Not technically minded here at all but had all the power we need for up to 7 weeks at a time with just the solar panel connected up to the 110 battery.

 

[ProfJohnL]

I have to agree with Martin, solar charging can be very effective, and can eliminate the need for a second battery, which also reduces the weight you have to carry.

I cant support Woody's method in the context of the OP's question:- A used older battery will have lost some of its ability to be charged to its full stated capacity. This is usually shown by the batteries unloaded terminal voltage, which will be lower than a fully charged new battery. If you connect these two batteries together in parallel, the higher voltage battery will try to discharge to the lower battery. as these battereies are lead acid accululators, they can produce some very high currents, which can certainly heat up even thick cables and possibiliy damage either the cable or the batteries. At worst it coudl be possible that the combination of chemical reactionand teh heating effect of a current through a conduictor coudl cause one or both batteries to explode.
That is why when ever paralell battery systems need one battery replacing, it is recommended practice by most manufactures to replace both batteries at the same time.

 

[JTQ]

I addressed the same requirement of increased battery capacity in a different way that does not involve any risk of battery interaction and matching. It might give food for thought as an alternative for the OP.

I carry only on the occasions when I need to, a second battery, a portable 65 Ah one; this is used for the demanding TV, satellite system & DVD player. I can select whether the TV etc is sourced from the van or the second battery
My 85 Watt solar cell’s controller is the readily available type able to charge two independent batteries. Mine is an expensive Morningstar duo, but a cheaper [£25 ish] similar option is available from Maplins.
This controller services preferentially the caravan battery at a 90/10% split and when that battery is full it attends to the needs of the portable battery. If I don’t carry the second battery then the controller recognises this and automatically sends all the solar panels out put to the caravan’s battery. I have also a set up to recharge the portable battery in transit within the car but apart from doing proving trials of the setup I have not needed to use it

I therefore have a solution where I can on trips where I need to store the excess yield from my solar cell for use specifically on what is my biggest power demand; the entertainment system. It does not inhibit the use of the van’s battery for this or in anyway allow interaction between the batteries. If I don’t need to take it the system looks after that. And the extra weight is transported within the car. The 65 Ah battery is more than adequate and quite readily portable; mine is in a weather proof enclosure with its own Aldi charger for servicing at home.

 

[Coops]

Many thanks peeps. Exactly the information I needed.
JTQ. Thanks to you for pointing out my obvious solution, as my regulator has the same (forgotten !) facility. My aims are similar to your own. Many thanks.