Secondary Battery

[Louey]

I'd guess any jump start cable would be fine? I'm also going to go for as big a battery as possible.

not always, depends on the quality of the jump leads - some are pretty poor. You get what you pay for with them.

[ghost123uk]

You don't need jump lead sized cables. The most current you will ever see in the main feed cable to the leisure is around 40 Amps, and even then that will only be for a few seconds (on start up), and then only if your leisure is seriously depleted. Look at the thickness of the main red wire that comes from the back of the alternator, that type of size is fine, and in fact a size down would also be fine.

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[nordberg]

I have been informed that if you are linking two batteries to effectively double your amp hours that they should be the same battery type from the same manufacturer and form the same batch because if there are any difference one will start drawing from the other? I'm no expert but could do with some feedback on this before i remove my perfectly good leisure battery to be replaced with a new one (or two as the case may be). Sorry to thread jack.

[ghost123uk]

I have been informed that if you are linking two batteries to effectively double your amp hours that they should be the same battery type from the same manufacturer and form the same batch because if there are any difference one will start drawing from the other?

Not really a worry, unless one of the batteries is on it's last legs. If you charge them both up, preferably on a "smart" charger, until they are full, then leave them for 24 hours, then measure the voltage on them, as long as this "resting" voltage is within .2 of a Volt, they will be fine together.

As some will know, I fly proper engine powered model aeroplanes. We use a hand held 12 Volt starter unit to fire them up on the field. For this purpose I use a small motorbike type battery. In the summer I keep this connected in parallel to my leisure battery (when not camping up) so it is always fresh and fully charged. So, I have a 100Ahr leisure battery and a (quite old but good) 17Ahr battery connected together. I have never experienced any issues at all with this, and I use my leisure battery very often for a wide variety of things, Propex almost daily in the winter, model battery charging, lighting, radio, laptop, etc etc.

[CJH]

I agree with Ghost, but I was equally worried about mixing battery types at first, because of first-hand experience - allow me to show you my photo of a pair of boiled batteries again



However, for the last couple of years I've also been running two completely different batteries in parallel for my leisure supply - one 'leisure' battery and one 'starter' battery, and I've not had any problems. Provided they are the same chemistry, and they 'live' together (i.e. are used in parallel and are charged in parallel), you should be fine. A few things (and a couple of 'rules') to consider:

1) Batteries linked in parallel will tend to settle to the same voltage - i.e. current will flow between them if there's a voltage difference, and this will reduce the charge of one and increase the charge of the other. Because of this, there can be a lot of current flowing between them if one battery is fully charged and the other is flat. It's good practice to make sure they are both fully charged before you link them together.

2) If you link one fully charged battery and one flat battery and then connect a smart charger, the smart charger will 'see' a combined voltage somewhere between the two, and will think it's connected to a single battery that needs a 'bulk' charge rather than a full battery that needs a 'float' charge. It will set a high charging voltage accordingly. But the fully charged battery should not have such a high voltage applied, especially if the charger is a high current charger, and it may boil. That's what caused the near catastrophic failure of the batteries in my photo - two batteries with dissimilar states of charge, connected to a 40A charger. So only charge the batteries in parallel if they're at a similar state of charge initially.

3) If the resistance in the wiring to the two batteries is not the same, they will drain and charge at different rates. One will 'age' faster than the other. There are diagrams on the internet for optimal wiring of two, three, multiple batteries in a bank. In my setup, not only are my batteries dissimilar, but the wiring is of different lengths. I've decided to accept different ageing rates - it's not the end of the world.

[nordberg]

Lots of great info there guys. Thanks. Really appreciated