Batteries: Mixing old and new

[rgriffiths]

I am new to my boat and still working out what needs to stay and what needs to go. There are 2 leisure batteries on board but I am not convinced that they are in the best of shape as they run out of charge quite quickly, but they do work . I want to change it to a bank of 4. If I were to buy two new ones is there anything wrong with connecting with the old ones or should it be a case buying 4 new ones?

[matty40s]

The older ones will be more likely to develop a fault, when this occurs, it will drag the whole bank down.

You are also likely to either not fully charge the new ones, or overcharge the old ones.

I have mixed in the past with no problems, but you do need to keep an eye on them to make sure all is OK.

[mrsmelly]

In short. Bin them. The 2 bad ones will bring the two good ones down to their level. Batteries are expendable consumables like diesel. I bought this boat two weeks ago. The sales bumff said it had 4 leisure batteries, I didnt even look at them. I bought 4 new ones and took them with me when I picked the boat up, first job before even moving off was to change the batteries. Having looked at the ones I took off it was a good move anyway, they were shot.

Buy cheap try to be good at charging but dont get anal about it. I am happy with 2 years and often get three from a cheapo set.

 

Tim

[Guest]

I'm in the 'change them all camp - get four new ones' too.

 

You don't know the history of the older ones, best to get shot.

 

Make sure you measure the physical size and check the positions of the pos and neg posts to get like 'like for like' so that they not only fit but also you dont have to faff about with the wiring.

 

PS welcome to the forum.

Edited February 3, 2015 by The Dog House

[nicknorman]

There is nothing inherently wrong with mixing old and new. Old batteries with reduced capacity will simply act like smaller batteries - taking less during charge, and delivering less during discharge. However, as Matty says (and depending on just how old) batteries can eventually fail due to debris collecting at the bottom shorting a cell or two, and if that happens it will pull the whole bank down and/or overheat and possibly even explode. Therefore it depends on how old and knackered they are, how you price peace of mind vs the replacement cost of new batteries.

[matty40s]

Which area are you in??

[dmr]

Matty is right here.

 

Electrically its fine to mix old and new, they will work together and whatever capacities they have will add togeher

BUT old batteries will fail at some stage and this might be catastrophic (and dangerous). with the healthy one discharging into the failed one.

Trouble is if you mix old and new you will loose track of just how bad the bad ones are, and likely forget just how old they are.

So I would say the only time you should mix old and new are....

1 A relatively new battery fails and needs replacing.

2 Money is tight and you can only afford 1 or 2 this month.....but make sure you replace the others next month.

 

If in doubt replace the lot.

 

............Dave

[Theo]

In short. Bin them. The 2 bad ones will bring the two good ones down to their level. Batteries are expendable consumables like diesel. I bought this boat two weeks ago. The sales bumff said it had 4 leisure batteries, I didnt even look at them. I bought 4 new ones and took them with me when I picked the boat up, first job before even moving off was to change the batteries. Having looked at the ones I took off it was a good move anyway, they were shot.

Buy cheap try to be good at charging but dont get anal about it. I am happy with 2 years and often get three from a cheapo set.

 

Tim

 

I bought seven cheapo ones last May. I normally only discharge them to 50% soc but on 2 or 3 occasions they have gone down below that. They now appear to be shot and I am thinking that buying cheap was a false economy. No time to find a link at the moment but there is a longish topic about my difficulties.

 

N

[matty40s]

 

I bought seven cheapo ones last May. I normally only discharge them to 50% soc but on 2 or 3 occasions they have gone down below that. They now appear to be shot and I am thinking that buying cheap was a false economy. No time to find a link at the moment but there is a longish topic about my difficulties.

 

N

we all responded lots Theo, did you ever come back to us??

[mrsmelly]

 

I bought seven cheapo ones last May. I normally only discharge them to 50% soc but on 2 or 3 occasions they have gone down below that. They now appear to be shot and I am thinking that buying cheap was a false economy. No time to find a link at the moment but there is a longish topic about my difficulties.

 

N

 

Hi

 

I dont think I ever go as low as 50 percent but I still think you have been unlucky. I usualy when off grid expect 2 winters out of mine. I change them as soon as I notice an increase in charging time as even cheap diesel as now soon adds up. I also think it depends on useage down to say 50 percent, such as wether that is having them screaming in agony being tortured through an inverter on a big draw or gently down just with say 12 volt lights and pumps etc.

 

Tim

[by'eck]

There is nothing inherently wrong with mixing old and new. Old batteries with reduced capacity will simply act like smaller batteries - taking less during charge, and delivering less during discharge. However, as Matty says (and depending on just how old) batteries can eventually fail due to debris collecting at the bottom shorting a cell or two, and if that happens it will pull the whole bank down and/or overheat and possibly even explode. Therefore it depends on how old and knackered they are, how you price peace of mind vs the replacement cost of new batteries.

 

In an ideal world where all battery types had the same voltage setpoints, and where they aged simply by loosing their capacity, your initial comments would be true, assuming you meant mixing them in a parallel connected bank of course.

 

In reality there are many reasons where one type would drag another down over time if not left on permanent float charge, even if both types were new, and creating the possibility of early sulphation for the newer ones.

 

Mixing old and new brings the issue of electrolyte and plate changes with age varying effective resistance and self discharge rates for example. Any failing battery creating excessive heat under charge may depress charge voltage if the device has temperature sensing, so falsely lowering it for the good batteries.

[matty40s]

 

In an ideal world where all battery types had the same voltage setpoints, and where they aged simply by loosing their capacity, your initial comments would be true, assuming you meant mixing them in a parallel connected bank of course.

 

In reality there are many reasons where one type would drag another down over time if not left on permanent float charge, even if both types were new, and creating the possibility of early sulphation for the newer ones.

 

Mixing old and new brings the issue of electrolyte and plate changes with age varying effective resistance and self discharge rates for example. Any failing battery creating excessive heat under charge may depress charge voltage if the device has temperature sensing, so falsely lowering it for the good batteries.

best not get too technical, the poor bloke has only just joined the fray.

[John Williamson 1955]

Time for the executive summary?

 

Don't mix old and new batteries, and buy them as matched sets, as differing makes and differing type numbers, even if identical capacity within makes don't always mix well.

[Loafer]

Time for the executive summary?

 

Don't mix old and new batteries, and buy them as matched sets, as differing makes and differing type numbers, even if identical capacity within makes don't always mix well.

That is indeed in the bible somewhere.

[Arthur Brown]

Charge the two that you have properly, then decide whether to buy two new ones scrapping the old ones or four new ones scrapping the old ones and rebuilding the battery retainers and links.

 

Don't mix ages of batteries it always ends badly.

[dmr]

Alternative executive summary....

Best not to mix old and new, but acceptable as long as batteries are of the same type (chemistry) and as long as you make sure to Never let any batteries get past their "die by" date.

 

A brand new set of identical batteries will show some cell to cell variation so no point in going overboard on "matched sets".

 

...............Dave

[peterboat]

When in the forces we had large battery banks and if one battery went down one was changed we used to have a good regime on battery maint ie emptying and washing batteries out refilling with fresh acid and desulphation charging. It did work we had batteries 20 years old still giving good service according to the charging logs. Now these batteries were high quality batteries very heavy and clearly well maintained. Bring on your bank have they ever received any maintenance are they cheapo do they even have any acid in them. I have a full traction set up and expect to get 20 years out of them why because with batteries spend more get more and maybe save money in the long run. So I would have a look at keeping your batteries alive for as long as possible whilst saving for some Trojans or better still 2volt cells

 

Peter

[Naughty Cal]

It is however still possible to kill expensive batteries, so unless you are going to ensure they are properly maintained and charged then you may as well stick with El cheapo batteries. They won't break the bank when they need replacing.

[ROBDEN]

In short. Bin them. The 2 bad ones will bring the two good ones down to their level. Batteries are expendable consumables like diesel. I bought this boat two weeks ago. The sales bumff said it had 4 leisure batteries, I didnt even look at them. I bought 4 new ones and took them with me when I picked the boat up, first job before even moving off was to change the batteries. Having looked at the ones I took off it was a good move anyway, they were shot.

Buy cheap try to be good at charging but dont get anal about it. I am happy with 2 years and often get three from a cheapo set.

 

Tim

 

Welcome.

 

The best advice? IMHO, from Mrsmelly.

 

Rob....

[Dinz]

You say boat is new to you.

If batteries replaced for new you know that power is good for sorting out any other problems.[if any]

However if charging and maintaining is one of the problems that MAY exist as you get to know the boat then you run the risk of goosing a new bank.

As a compromise why not just fit 2 new and then check that everything else is ok?

Then adding a pair in a month or so if all is good.

If system is ok the first pair will still be healthy.

If their is any problems you will only knacker 2.

Charging side may need upgrading for larger bank.

[rgriffiths]

Thanks to everyone for all the informative replies. It sounds like the best plan is to replace.

Which area are you in??

Victoria Park in London.

[Naughty Cal]

At least that way you are starting afresh.

[bottle]

rgriffiths

 

If you get this far, welcome.

 

If you get four new ones, then have a look here on how to wire them for best service.

 

While you are at it check that the cables, interlinks etc. are of substantial size, I would suggest 50mm² as the very minimum but it will depend on length.

 

Do come back for more advice if you need it.

[matty40s]

Thanks to everyone for all the informative replies. It sounds like the best plan is to replace.

 

Victoria Park in London.

Look for a local automotive battery supplier, they will be far newer, and maybe filled to order rather than having stood on a chandlery floor for 6 months.

[Steve Hill Marine]

I am new to my boat and still working out what needs to stay and what needs to go. There are 2 leisure batteries on board but I am not convinced that they are in the best of shape as they run out of charge quite quickly, but they do work . I want to change it to a bank of 4. If I were to buy two new ones is there anything wrong with connecting with the old ones or should it be a case buying 4 new ones?

You should never mix old and new, Alternators and battery charger will read the weakest batteries when deciding how much charge to generate, therefore overcharging the new ones. Always change the lot at the same time, for the same brand and amp/hour rating. Steve Hill Marine.