Parallel leisure batteries

Hi,

I have an application (a VHF repeater) that requires 12V for about a week when it's set up once each year. We've been using a leisure battery but this needs changing halfway through the week.

I'm looking for a way to run multiple batteries without needing to physically swap them over (the repeater is in a tricky location).

Firstly, is it possible (and safe) to run leisure batteries in parallel? My internet research says that this is common practice and that for well-matched batteries (same capacity, same level of charge) this is fine, as long as each battery is fused individually. I'm aware that common practice isn't often best practice.

I need to make the set-up idiot-proof, and was thinking about making a battery select circuit that, say, connects the load to the battery with the highest terminal voltage.

I am aware that a couple of diodes would be a simple solution, but I'd prefer a more efficient approach that doesn't drop the battery voltage, even by the 400mV of a Schottky (at 8A or so, during transmission).

Also, 400mV x 8A is 3.2 Watts so heat sinking becomes an issue, especially in the worst case scenario of an open mic causing the repeater to transmit continuously.

I was thinking of using a couple of high-side P-channel MOSFETs, with one or other selected by a comparator (or even a microcontroller, which could be a bit more flexible) comparing terminal voltages. Some hysteresis (and/or time limiting) will be necessary to stop the on-load voltage drop from causing the thing to oscillate between batteries.

Does anyone have any suggestions, or pointers to existing circuits, to manage two (or possibly more) batteries in some way? I would imagine it's a common problem but Internet searches haven't revealed much. Maybe people just put the batteries in parallel and have done with it.

Thanks in advance,

Andrew

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Reply to
Andrew Feldhaus
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"Andrew Feldhaus"

** Boy you are dumb.

All you need is an 84 hour (ie 3.5 day ) timer and a change-over relay.

Do I really have to spell it all out for you ??

... Phil

Reply to
Phil Allison

Yes. Lead-acid batteries are well suited to run in parallel. The same is true for lithium based batteries.

Whether to fuse or not, depends on the application. In automotive applications, the there is no fuse between batteries or alternator.

It is correct that the paralleled batteries should as perfectly matched as possible. Ideally, buy them both at the same time. When one of them dies, replace both.

On the other hand, if it has been done successfully a million times, it might just be.

The best route to idiot proof goes through simplicity. Some say simplicity is the ultimate sophistication.

If all you need is more capacity, parallel connection is the right choice.

If you need some kind of redundancy or reserve, you will need more complexity. For example, camper vans have two batteries which are both charged by the engine's alternator, but the camping equipment only drains one of the batteries. That way, you'll be able to start the engine even if you left the lights on all night.

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RoRo
Reply to
Robert Roland

What is a "leisure battery" ?

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   
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Reply to
John Larkin

deep cycle meant for caravans and such, not starting a car

-Lasse

Reply to
langwadt

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It has a rather laid back discharge curve...

mike

Reply to
m II

Sno-o-o-ort ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

It is extremely common to parallel leisure batteries, and also common for them to be of different ages and makes with no ill effects.

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Mike Perkins
Video Solutions Ltd
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Reply to
Mike Perkins

Golf cart batteries and fishing batteries would qualify.

Greg

Reply to
gregz

Vibrator?

Reply to
krw

gregz wrote in news:2072573027363560067.341828zekor- snipped-for-privacy@news.eternal-september.org:

IOW,"deep discharge" L-A batteries.

Maybe they are those small 12V SLAs used in many motorized scooters,the ones the size of a motorcycle battery(17-20AH),and that's why he wants to parallel them?

or maybe you just recharge them at your leisure.... ;-)

Why doesn't the guy get a 12V solar panel to keep his repeater battery charged at remote sites? low maintenance,"free" power.(TANSTAAFL....) I see them on poles in many school zones,used to power flashing yellow LED lights while school crossing guards are on duty.

solar panels are available in several different wattages. He'd probably need a charge controller with anti-sulfation circuitry.

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Jim Yanik
jyanik
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Reply to
Jim Yanik

You can Parallel Deep-Cycle batteries. But you should follow the general rule that they be the same capacity , age and voltage. This is probably your best route.

Cheers

Reply to
Martin Riddle

That is about all they are any good for ;-) We have them in the UK powering "please go round the dangerous bend" signs. They only work in mid-summer and are always dead in the water on frosty winters mornings when they are actually needed. And they don't work either - here is a car that has just passed one of these active signs parked in the living room of a house after failing to negotiate the "dangerous" bend!

formatting link

And around 100W or more worth of solar panels if it has to supply 8A peak load to the transmitter for most of the day. But you are right - this is one example off grid where a solar panel despite being bulky might have the price/weight/convenience performance to be better than the alternative of lugging a second heavy battery up a mountainside.

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Regards,
Martin Brown
Reply to
Martin Brown

Check out LTC4412 and similar devices. Designed just for this type of job.

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Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
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Reply to
Adrian Jansen

Hi,

Thanks for all of your replies and suggestions. I'll address a few of these in one place, for ease of reference:

Firstly, yes, a leisure battery is (effectively) a deep-discharge, high capacity, low power version of a car battery. They're particularly commonly used in caravans and mobile homes for powering TVs and whatnot.

Timer + relay:

This would work but doesn't compensate for the muppet factor of forgetting to charge one battery fully (or it failing altogether). Also, compared to the quiescent load of a non-transmitting repeater (A few hundred mA) holding a relay with (typically) 1W is a little wasteful (certainly compared to a MOSFET), though I respect that a latching relay could be used instead.

Paralleling batteries:

Thanks to those who pointed out that, if carefully done, this is ok. If time is short this is probably what we'll end up doing though I will be aware that matching is an issue. Muppetry and/or premature battery failure (we've already had one!) would make life interesting.

Solar:

Yes, this is something we've considered but thanks for reminding me. The repeater is actually used on successive nights and left virtually idle during the day. I suspect even the British summer could provide enough power to keep it topped up. I will investigate this further.

LTC4412:

Thanks, in particular, to Adrian Jansen who points out this handy chip. I had thought there had to be an existing solution and this looks like it. In the context of the original circuit idea, this IC saves a great deal of hassle.

Once again, thanks to you all.

Andrew

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Reply to
Andrew Feldhaus

if your solar panel is enough to charge the battery in two weeks it is big enogh to make a battery that would last only one week last two weeks.

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Reply to
Jasen Betts

Not this year it wouldn't. I have forgotten what the sun looks like. A cheap 20W solar panel might be worth a punt as a lifetime extender. But at present UK daylight levels it would deliver only about 3W.

Puts about 12Ah per sunny day back into the battery so depending on your nightly average transmit might make a worthwhile contribution.

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Regards,
Martin Brown
Reply to
Martin Brown

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