matching batteries

[dodgy01]

i understand if you have 2 knackered ones and you only replace 1 of them , but if you only have 1 failed battery why on earth would you replace both

[dodgy01]

also when they are being charged the main relay is energised so basically the batteries are wired in parallel so the alternator can't tell if there is 1 or 21 batteries

[bambi mk 1]

I’ve been running not matching batteries of differing age for over 4 years, dragging high amps out them with inverters etc and had heehaw bother at all
But then? What do I know?
I just cut wood

dont confuse luck with knowledge.

You have a mk6 tdci so you must have neither No problem changing one battery if wired parallel

[dodgy01]

[MK7 user]

ive been a mechanic for a fair while and now re trained as a electrician
please explain why batteries should be replaced as a matched pair
even fords dont replace them both at the same time if only 1 is at fault

Which post in this thread advocates they need to be changed as a 'Matched pair'

i understand if you have 2 knackered ones and you only replace 1 of them

So the 'penny dropped' eventually....I see you reside in a place called 'brain-tree'

Mixing old and new is a great way to kill new batteries.

+1.

Buying 2 new batteries when 1 is still good is a great way to waste money

+1....fitting a new one in parallel with a knackered one results in a similar waste.

[Brains_transit]

I had 2 in parallel and I knew one or both were failing and since I'm not a tight arse I got 2 new ones £140 trade. I seen 1 selling for £120 so I'm happy

[the dutch guy]

ive been a mechanic for a fair while and now re trained as a electrician
please explain why batteries should be replaced as a matched pair
even fords dont replace them both at the same time if only 1 is at fault

please point out my comment were i talk about matched pairs.

I would think its like a pack of lipo batteries where you balance charge them
Or the weak cell gets fried and the good cell gets undercharged
But its not as if 2x 12volt batteries have 12 individual cells or enithink init

a 12v SLA battery has 6 cells. and yes, just like lithium they need to be balanced to prevent overcharging.
SLA cells are by their nature fairly robust against overcharging as long as it remains sensible. but if a cell dies the load on the other cells increases and you kill the rest of the cells really fast.
if you use a lifepo4 battery (like i do) you can use balance boards on the battery to activly protect against overcharging and when a cell does die (you only have 4) you can really just replace the dead cell. much cheaper (in the long run), lighter,reliable and much more powerful then lead.

i understand if you have 2 knackered ones and you only replace 1 of them , but if you only have 1 failed battery why on earth would you replace both

that a battery is capable of starting the van does not mean it is still "good". you need to look at the resting voltage. if a worn battery has a float voltage of 12.2 for example (~25% capacity remaining) and you put a fresh battery with a float voltage of 12.7v next to it the good one will constantly discharge into the worn battery charging it but it does not hold the charge so it only heats up draining the new battery. this happens every time you turn off the van. this will almost fully cycle the new battery every day wich will kill it real fast. you can see this when you measure the current flow with a camp meter between the batteries when the engine is off.

also when they are being charged the main relay is energised so basically the batteries are wired in parallel so the alternator can't tell if there is 1 or 21 batteries

charging the batteries is not the issue here. the problems start when you turn off the engine.

[dodgy01]

ok so with out the cheap insults of where i live or used to live

i never said anything about putting a good battery with a knackered battery
i said why replace both batteries if only 1 has a fault and as yet you have not been able to give me a reason , just because both batteries are not new does not mean both have faults

and how is the issue when you turn off a engine
the relay separates the batteries once the engine is not running , so what your saying about 1 battery draining the other battery is not even possible

[the dutch guy]

the relay separates the batteries once the engine is not running , so what your saying about 1 battery draining the other battery is not even possible

that depends on your setup. plenty of people just do a double starter setup.

ps: nowere in my post i am trying to insult you.

[MK7 user]

the relay separates the batteries once the engine is not running , so what your saying about 1 battery draining the other battery is not even possible

that depends on your setup. plenty of people just do a double starter setup.

+1.

If you don't mind my saying Mr dutch guy, I think you are 'pissing in the wind' with this one.

The fact that your comments have at sometime in the past become a 'Scientifically proven fact' by an 'Eminent Scientist' (no not bjt) is lost on your audience.

I'll take the opportunity to wish yourself & your family a Merry Xmas & a Happy & Prosperous New Year.

[dodgy01]

the relay separates the batteries once the engine is not running , so what your saying about 1 battery draining the other battery is not even possible

that depends on your setup. plenty of people just do a double starter setup.

ps: nowere in my post i am trying to insult you.

it was aimed at mk7 mate , not you

[the dutch guy]

it was aimed at mk7 mate , not you

apologies for misreading your post.

[dodgy01]

no need bud