Vara
Well-Known Member
The rule I applied is as big as you can fit and afford. Surplus battery capacity is no bad thing.
3x80 domestic, 1xvlarge starter, which came with boat.
3x80 domestic, 1xvlarge starter, which came with boat.
Batteries....what rating?
- Thread starter Tryweryn
- Start date
ffiill
Well-Known Member
3x110 ah one dedicated to start the other two domestic but in emergency I can put any or all to start.
alan_d
Well-Known Member
Yup, buy the biggest you can afford that will fit in the space. (Don't forget to check the height as well as length and breadth, and the position and type of the terminals.) The less (proportionately) a battery is discharged the longer it will last.
lpdsn
Well-Known Member
I agree with alan_d & vara. The bigger capacity you can afford & fit the better.
Compare the weight & dimensions for the various batteries you are considering. Some manufacturers claim more Ah for the same amount of Lead with the same basic design. I suspect that rather than their designs being more economical with the lead, their Marketing Depts are being more economical with the truth.
And...[hobby-horse] you don't need a switch to parallel up the domestics and a duff starter, you need switches to switch out the duff starter and switch in the domestics to start the engine in an emergency. Paralleling up a duff battery with your good batteries will drain capacity from the good batteries before you've even attempt to start the engine. [/hobby-horse]
JumbleDuck
Well-Known Member
'Swhat I did. Unfortunately my battery locker is quite shallow, so the biggest I could fit there was 2 x 65Ah for domestic, and I added a shelf for a small engine start battery (an 063 I had lying around). On the bright side, my electrical needs are minimal - LED lighting, simple GPS, no plotter, occasional radio, netbook charging - so 130Ah is more than enough.
JumbleDuck
Well-Known Member
As anyone who has ever jump started a car hasn't discovered ....
prv
Well-Known Member
You would normally have the donor car running, though.
Although anyone who thinks that all of my 420Ah domestic bank is going to empty itself into the 28Ah engine battery is sadly deluded, I think
Pete
ghostlymoron
Well-Known Member
You usually jump start with the donor cars engine running! I'm with lpdsn on this but I've been shouted down before.
Bav34
...
2 x 220 domestic means we can run the fridge for > 8 days in 25 degree weather plus lights, water pumps etc
Can't remember what the engine battery is. Adequate
Can't remember what the engine battery is. Adequate
lpdsn
Well-Known Member
Although anyone who thinks that all of my 420Ah domestic bank is going to empty itself into the 28Ah engine battery is sadly deluded, I think
Pete
Maybe if there was a cell gone...
Anyway, it costs a few quid for an extra battery switch (maybe not even that if you already have a link switch) to get a better arrangement.
It's what I've done and at absolute worst I'm happily deluded.
ghostlymoron
Well-Known Member
But most? people have something like 85ahr domestic and 55ahr starter so are not deluded. Plus you want in an emergency to give yourself the best chance of starting you can.
You would normally have the donor car running, though.
Although anyone who thinks that all of my 420Ah domestic bank is going to empty itself into the 28Ah engine battery is sadly deluded, I think
Pete
JumbleDuck
Well-Known Member
You would normally have the donor car running, though.
Although anyone who thinks that all of my 420Ah domestic bank is going to empty itself into the 28Ah engine battery is sadly deluded, I think
I've done it many times (classic cars ...) using just a battery, and have never had problems. I also have one of those starter packs with a small gel cell cell in it, and that doesn't seem to get flattened when used to start something.
Long term I dare say it could be an issue, but the titchy little spark you get connecting jump leads suggests to me that there really isn't enough current to worry about over the time scales needed to start an engine.
PetiteFleur
Well-Known Member
2 x 110ah, with old fashioned off/batt1/batt2/both switch. Use one for starting and domestics, usually charged during the day, then use 2nd for next day, alternating over a cruise, with fridge - works for us. Small 5A Ctek battery charger for when on shorepower.
Tryweryn
Well-Known Member
Went for 2 x 110ah domestic and the starter is fine. many thanks for all the info. Changing to LED lights etc must help. This however was counteracted by a flipping TV lol
ghostlymoron
Well-Known Member
If you've got 10v in your 55ahr starter and 12v in your 85ahr domestic and you parallel them you'll have only 11.2v available to start your engine which may not be enough. Tell me where my logic/math is wrong.
RichardS
N/A
macd
Well-Known Member
Because the smaller (55Ah) battery will sway the average less than the larger (85Ah) one. At least, I presume that's ghostly's thinking. I doubt it works in practice precisely like that, but the effect will probably lean that way. I'd add that if the 85Ah job is actually showing 12V, whoever looks after it has been a little careless since that's over 50% discharged. But it happens.
Artic Warrior
Well-Known Member
MMMMmm well it is if you cant charge them high enough, and they remain low and never recover that's the problem
JumbleDuck
Well-Known Member
I don't know what your maths is! It looks as if it may be (10 x 55 + 12 x 85) / (55 + 85) but I can't see any physical basis whatsoever for that calculation, so I presume that's not it.
ghostlymoron
Well-Known Member
That's correct Jumbleduck. Its the hydraulic analogy. Ahrs = volume, voltage = depth(head). Seems logical to me. I suppose you could prove the theory by connecting two unequally charged batteries together and measuring the combined voltage.
