Fit another leisure battery

[B J McKay]

I have an Autotrail Mohawk with space and wiring for a second leisure battery. Can I just fit another battery and connect it to the wires already there .

 

[Pudsey Bear]

In principle yes but they are better as a matched pair, same make and capacity for the best result, at least the same capacity, but the older on will be part depleted.

 

[Rumour]

We added one to our two older batteries. We bought an identical battery (Hankook XV110MF) and added it via the inverter wiring, after removing the inverter. It did increase the capacity of storage, but as the original two batteries were four years older, it just didn't seem to make the difference we thought it should. Had all three batteries checked independently, off the van and found that although they showed full charged in the little widow on the battery, the capacity or something of the older units had fallen. (I really didn't understand what he said!) The newer one seemed to be still OK.

As the Hankook was no longer available, we spoke to Tayna Batteries and they pointed me in the direction of an almost direct replacement. We bought three. All fitted and we can see the difference. Starting the Autoterm D5 Flow heater, the battery voltage dropped to around 12 volts, sometimes lower! With the new batteries, at night and freezing temperatures, it doesn't go below 12.8 volts. (It recovers a bit once started.) Not sure if this is normal, but it certainly made a difference for us.

 

[Trotter]

I have an Autotrail Mohawk with space and wiring for a second leisure battery. Can I just fit another battery and connect it to the wires already there .

Fogstar. You’ll never regret it. Take full advantage of the offered discount.Total game changer.

 

[merl]

with space and wiring for a second leisure battery

As long as both batteries are the same chemistry/type you'll be fine. So dont mix AGM, GEL, Lithium.

 

[wildebus]

As long as both batteries are the same chemistry/type you'll be fine. So dont mix AGM, GEL, Lithium.

I wouldn't agree with don't mix Lead (i.e. AGM, GEL) and Lithium comment Can work very well if done right! (I've have Lead and Lithium in parallel for over 3 years in my Motorhome).

What I WOULDN'T mix is aged Lead with new Lead. That can be a false economy and not give you whay you might expect.

 

[merl]

I wouldn't agree with don't mix Lead (i.e. AGM, GEL) and Lithium comment Can work very well if done right! (I've have Lead and Lithium in parallel for over 3 years in my Motorhome).

What I WOULDN'T mix is aged Lead with new Lead. That can be a false economy and not give you whay you might expect.

Of course it's possible to mix lithium and lead with the right addition equipment. Thing is though David the OP is adding his new battery onto the existing wiring that's already in place, the OP should not fit simply add a lithium battery in that position.
WRT not mixing aged batteries with new, again I've posted on this topic before, it's perfectly acceptable and not massively detrimental. A batteries required charge profile doesn't change with age, once you've fitted an AGM or GEL battery your charger stays set at AGM or GEL and you don't change that so if you've got an old battery that lets say has aged to only 70% capacity and you add a new battery with 100% capacity when those 2 batteries are wired in parallel their voltages will be locked together and identical, they will now receive the same charge voltage (just like they would IF they were not wired in parallel) so they'll both become funny charged at the same time. They will also become 'flat' at the same voltage (around 12V OCV) so effectively the user will use both batteries to their maximum available capacity ie 170%, if that 170% is less than "expected" then that's the fault of the expectant, not the batteries or wiring.
What IS important is not overcharging or undercharging and that's why it's important to match the chemistry NOT the age. Keeping the AH the same shouldn't really matter either for the reasons above. As for keeping to the same brand.... well that's just nonsense spouted by dealers and 'forum experts' with little electrical/electronics knowledge repeating poor advice, absolute tosh and urban myth.

 

[Trotter]

Of course it's possible to mix lithium and lead with the right addition equipment. Thing is though David the OP is adding his new battery onto the existing wiring that's already in place, the OP should not fit simply add a lithium battery in that position.
WRT not mixing aged batteries with new, again I've posted on this topic before, it's perfectly acceptable and not massively detrimental. A batteries required charge profile doesn't change with age, once you've fitted an AGM or GEL battery your charger stays set at AGM or GEL and you don't change that so if you've got an old battery that lets say has aged to only 70% capacity and you add a new battery with 100% capacity when those 2 batteries are wired in parallel their voltages will be locked together and identical, they will now receive the same charge voltage (just like they would IF they were not wired in parallel) so they'll both become funny charged at the same time. They will also become 'flat' at the same voltage (around 12V OCV) so effectively the user will use both batteries to their maximum available capacity ie 170%, if that 170% is less than "expected" then that's the fault of the expectant, not the batteries or wiring.
What IS important is not overcharging or undercharging and that's why it's important to match the chemistry NOT the age. Keeping the AH the same shouldn't really matter either for the reasons above. As for keeping to the same brand.... well that's just nonsense spouted by dealers and 'forum experts' with little electrical/electronics knowledge repeating poor advice, absolute tosh and urban myth.

Been there, done that. I do know that the setup I have now, is better than anything I’ve had before.
But then again, I’m only a numpty. Theories as much as you like. The real world is different and sometimes simpler to follow the advice of people you trust.

 

[merl]

Been there, done that. I do know that the setup I have now, is better than anything I’ve had before.
But then again, I’m only a numpty. Theories as much as you like. The real world is different and sometimes simpler to follow the advice of people you trust.

I think that most people's current system is the best they've ever had Del, including my own system, that's progress for you.

 

[wildebus]

Of course it's possible to mix lithium and lead with the right addition equipment. Thing is though David the OP is adding his new battery onto the existing wiring that's already in place, the OP should not fit simply add a lithium battery in that position.

That is very true, and the information on Lithium being a simple "Drop In" battery to replace Lead is only true if you van is "lithium-capable" already. An Autotrail Mohawk will not be.
The last battery I fitted in a Motorhome was actually a Lead-Acid. Bought a 200Ah Lead Carbon GEL Battery to replace a pair of worn-out AGM Batteries in a Murvi. Could have very easily got a Lithium Battery for the same money with no problem, but the budget (and the requirements) didn't make Lithium a worth-while switch.

But you made a bit of a blanket statement ref mixing types and I am responding to that

WRT not mixing aged batteries with new, again I've posted on this topic before, it's perfectly acceptable and not massively detrimental. A batteries required charge profile doesn't change with age, once you've fitted an AGM or GEL battery your charger stays set at AGM or GEL and you don't change that so if you've got an old battery that lets say has aged to only 70% capacity and you add a new battery with 100% capacity when those 2 batteries are wired in parallel their voltages will be locked together and identical, they will now receive the same charge voltage (just like they would IF they were not wired in parallel) so they'll both become funny charged at the same time. They will also become 'flat' at the same voltage (around 12V OCV) so effectively the user will use both batteries to their maximum available capacity ie 170%, if that 170% is less than "expected" then that's the fault of the expectant, not the batteries or wiring.
What IS important is not overcharging or undercharging and that's why it's important to match the chemistry NOT the age. Keeping the AH the same shouldn't really matter either for the reasons above. As for keeping to the same brand.... well that's just nonsense spouted by dealers and 'forum experts' with little electrical/electronics knowledge repeating poor advice, absolute tosh and urban myth.

Well, no surprise that I don't agree with you regarding simply adding an new battery in parallel to an old battery. Sometimes it can be ok depending on how much it has been used and how well looked after, but as a default position .... don't do it!

 

[merl]

But you made a bit of a blanket statement ref mixing types and I am responding to that

I didn't think I'd made a blanket statement, rather I thought I'd addressed the individual requirements of the OP who wanted to add an additional battery onto wiring that was already in place, that's why I copied in "with space and wiring for a second leisure battery" into the header


I cant envisage a common situation where adding a second battery to another partially depleted one wouldn't be of benefit let alone detriment?
Sure IF the first battery was essentially knackered with very low useable capacity and lugging it around and taking up space isn't worth the return or has a cell down then yes that battery needs replacing but under those circumstances I'd expect the OP would be posting about "which replacement battery to buy" and not "which battery to add"? I guess I should have added that caveat to the advice just in case and not assume the OP's existing battery still had useable capacity. If the existing battery has useable capacity then why not use it? I cant think of a science based argument for not doing so for the reasons already posted I wouldn't adopt a default position of "dont do it" but that's just my opinion.
I've seen posts regarding the old battery 'dragging down the new one' etc but I cant see (scientifically) how that happens? As I said, I'm sure there's some instances where peoples expectations haven't been met but that's hardly scientific evidence or proof, rather it's merely anecdotal and based on massively variable conditions and hence completely unreliable. I'd be prepared to put money on a capacity test showing a real world improvement so my default position would be "Go Ahead" with caveats regarding not expecting the old battery to 'improve' and need replacing sooner than the additional and like I said keep the chemistries the same.

Sometimes it can be ok depending on how much it has been used and how well looked after, but as a default position .... don't do it

More than willing to be edumacated if I've got the science wrong but please NO posts about "I fitted an extra battery and we only got X nights" etc, etc. I'm sure there's lots of people who've fitted a new battery and then had a puncture, that's not science that's anecdotal nonsense. I repeat
I'd be prepared to put money on a capacity test showing a real world improvement.
Takers please

 

[Trotter]

10 &11
Game on.

 

[wildebus]

I didn't think I'd made a blanket statement, rather I thought I'd addressed the individual requirements of the OP who wanted to add an additional battery onto wiring that was already in place, that's why I copied in "with space and wiring for a second leisure battery" into the header

In this thread, there were also posts about adding Lithium and it became more of a general discussion. You are old enough to know threads go off-track and people start reading them half-way through.

I cant envisage a common situation where adding a second battery to another partially depleted one wouldn't be of benefit let alone detriment?
Sure IF the first battery was essentially knackered with very low useable capacity and lugging it around and taking up space isn't worth the return or has a cell down then yes that battery needs replacing but under those circumstances I'd expect the OP would be posting about "which replacement battery to buy" and not "which battery to add"? I guess I should have added that caveat to the advice just in case and not assume the OP's existing battery still had useable capacity. If the existing battery has useable capacity then why not use it? I cant think of a science based argument for not doing so for the reasons already posted I wouldn't adopt a default position of "dont do it" but that's just my opinion.

I've seen posts regarding the old battery 'dragging down the new one' etc but I cant see (scientifically) how that happens?

It happens. I spoke to someone who had three batteries and one of those batteries was causing an overall voltage drop. I didn't see the motorhome myself so troubleshooted the issue via email but the guy got the battery setup working by removing the problem battery from the circuit.
call it anedotal if you want, but he saw it

As I said, I'm sure there's some instances where peoples expectations haven't been met but that's hardly scientific evidence or proof, rather it's merely anecdotal and based on massively variable conditions and hence completely unreliable. I'd be prepared to put money on a capacity test showing a real world improvement so my default position would be "Go Ahead" with caveats regarding not expecting the old battery to 'improve' and need replacing sooner than the additional and like I said keep the chemistries the same.


More than willing to be edumacated if I've got the science wrong but please NO posts about "I fitted an extra battery and we only got X nights" etc, etc. I'm sure there's lots of people who've fitted a new battery and then had a puncture, that's not science that's anecdotal nonsense. I repeat

I'd be prepared to put money on a capacity test showing a real world improvement.
Takers please

go and do it then. actions speak louder than words. stop waffling and get on with it

 

[merl]

stop waffling

So.... I'm the one using science and the laws of physics, and you're the one quoting some bloke or other that you don't know that once did something that you also don't know and can't confirm but somehow I'M the waffler??

 

[wildebus]

So.... I'm the one using science and the laws of physics, and you're the one quoting some bloke or other that you don't know that once did something that you also don't know and can't confirm but somehow I'M the waffler??

Actually, he found my help useful enough to post a review on Trustpilot ...

Myzzy Pro gave Wildebus 5 stars. Check out the full review...

I contacted David at Wildebus by email explaining the electrical issues I was experiencing with ...

www.trustpilot.com

Solar system and utility battery wiring & charging issues. I contacted David at Wildebus by email…​

I contacted David at Wildebus by email explaining the electrical issues I was experiencing with my Motorhome, David responded very quickly requesting all the issues I was experiencing to which I gave him. Again, his response was within the hour giving me his experienced opinion and a series of tests to perform to narrow down the cause. The results were pretty conclusive and from this I was able to easily resolve the issue myself without having to pay him a visit. Very knowledgable and experienced to be able to pinpoint the issue by email. Highly recommended and wouldn’t hesitate to contact him again if needed. Many thanks.

Why don't YOU show me what YOU have done in a similar situation?

 

[merl]

Actually, he found my help useful enough to post a review on Trustpilot ...

Myzzy Pro gave Wildebus 5 stars. Check out the full review...

I contacted David at Wildebus by email explaining the electrical issues I was experiencing with ...

www.trustpilot.com

Solar system and utility battery wiring & charging issues. I contacted David at Wildebus by email…​

I contacted David at Wildebus by email explaining the electrical issues I was experiencing with my Motorhome, David responded very quickly requesting all the issues I was experiencing to which I gave him. Again, his response was within the hour giving me his experienced opinion and a series of tests to perform to narrow down the cause. The results were pretty conclusive and from this I was able to easily resolve the issue myself without having to pay him a visit. Very knowledgable and experienced to be able to pinpoint the issue by email. Highly recommended and wouldn’t hesitate to contact him again if needed. Many thanks.

Why don't YOU show me what YOU have done in a similar situation?

Ooh I can't compete with that David. I'm afraid most of my electronic & electrical knowledge is based on an electrical part P qualification and designing and building amplifiers and repairing electronics, so nothing as complicated and technical as connecting two batteries together but I've got a letter from my mum saying I'm the nicest boy she knows, that any good?

 

[wildebus]

Ooh I can't compete with that David. I'm afraid most of my electronic & electrical knowledge is based on an electrical part P qualification and designing and building amplifiers and repairing electronics, so nothing as complicated and technical as connecting two batteries together but I've got a letter from my mum saying I'm the nicest boy she knows, that any good?

Not really, no. Some of us live in the real world with real issues to fix.
I also have electrical and electronic qualifications but so what.
Part P qualification? very handy for a motorhome

 

[merl]

Not really, no. Some of us live in the real world with real issues to fix.
I also have electrical and electronic qualifications but so what.
Part P qualification? very handy for a motorhome

Ner ner ner ner ner.

 

[Okta]

From my comfortable position on this fence I think there is room for anecdotal and scientific evidence concerning un-matched batteries. I hesitate to ignore anecdotal evidence when it can be found widely on motorhome, boat and solar websites. However it does lead me to look for the science that might possibly explain it.

In the days when fixed voltage battery slaying ‘chargers’ were the norm, having batteries of different ages and capacities might have gone largely unnoticed because neither would ever have achieved a fully charged status. Now we have multi stage chargers it raises the question of whether the clever technology can cope properly with batteries that are not matched. Some suggest that the older lower capacity battery will be fully charged before the newer one but the charger will continue to charge at the higher voltage until both batteries are full, during this period the already full one is in danger of overcharging which could damage it further.

Another possible problem area is charging efficiency. Probably not much of an issue when both batteries are being charged by either a mains or B2B charger but perhaps it does come into play when off grid and drawing power. The older lower capacity battery might discharge quicker than the new one and as its voltage drops the higher voltage one will try to charge it, alongside supplying the consumer demand. At this point charging efficiency might well become a factor because up to 20% of the power going into charging the old battery could be lost in charging inefficiency (heat), thus reducing the original total stored power. The Peukert Exponent might also be a small factor because the discharge efficiency of the good battery also decreases as the Amps rise. In this way there does seem to be some scientific basis for saying the older battery can drag the newer one down.

I will decline the blindfold when both sides of this discussion decide that they need to unite to form my firing squad.

 

[merl]

The nice thing about electronics is it's predictability, now I'm not talking about predictability of wear and tear, longevity, reliability and the like because these are variables that are far too nuanced and affected by myriad of mainly external influences that predicting them with any great accuracy is hard. When I say electronics is predictable I mean the actual flow of electrons through components and around a circuit is predictable, this is because electricity is bound by fundamental laws of physics, we understand those laws and those laws are so predictable that it can be modelled with near 100% perfect accuracy with a computer program. The reason why it's not absolutely 100% perfect is not because of unpredictability of how electrons act, it's down to the inability to factor in the minute detail of interactions, in a complex electronic circuit we can model exactly what's going to happen WRT all of the individual components but, depending on how elaborate the computer program is, there could be secondary interactions like stray capacitance and resistance but we're getting into an area where only complex high frequency electronics would be troubled, and certainly not when it comes to a simple task like modelling what will happen when two batteries of unequal capacity are connected in parallel. Is connecting two batteries together really a circuit? YES, it's a VERY simple circuit, probably second only in simplicity to connecting two batteries (or whatever) in series, so yes connecting two batteries in parallel IS an electronic circuit AND it can be modelled with very highly predictable results.
I have LT spice on my PC, I use it from time to time to model electronic circuits, typically filters because it's easier to use LT spice to model the results than go through the maths and do all the calculations. Have I modelled what happens when you connect 2 batteries of unequal capacity in parallel? NO I haven't because frankly I CAN do THIS maths conundrum in my head, it's exactly the same as connecting 2 capacitors in parallel.
It's Cx = C1+C2. So to find the new battery capacity (Cx) we add together the capacities of the other two.
Just like capacitors. two batteries connected in parallel charge equally, one battery can't charge before the other or discharge before the other because they are forced to do so because they are in parallel. The voltages measured at each battery will always be identical, if the chemistry is the same as each other they will therefore both reach full charge together and they'll both reach full discharge together. One battery DOESNT pull the other down or up or sideways! They sit side by side at equal levels, just like two tanks of water, they DONT affect each other.
So, if we have a battery of 100Ah and we parallel this with a battery of 50Ah capacity we get a new total capacity of...yes you guessed it...150Ah. It doesn't matter how old either battery is, or who made them or which one's had the harder life or which one was blessed by the Pope, it doesn't matter, connect 100Ah in parallel with 50Ah and you'll get 150Ah, in fact you should get slightly more because if you've conducted the capacity tests of all three batteries at the same constant current then Peukert's law comes into affect. The newer battery WILL NOT suffer WRT ageing because it's paralleled with an older one, in fact it's easy to see that it's aided by the other so it's cycle depth will decreased and should age slower than it would have done if it were working by it's self.

Batteries must be of similar type/chemistry, if not one battery could be over/under charged.
Yes, wiring resistance between the batteries could/will skew the results, but if the wiring resistance is that high that it does then that wiring needs altering. (And I might add that if the batteries are connected together with a significant resistance then those batteries are NOT technically in parallel)
I'm not talking about or including shagged batteries, batteries with a cell down et al, like I said we're talking about batteries with sensibly useable capacity.
Obviously when paralleled up the older/most aged battery is likely to fall into the red group above before the other one, when it does, ... see what's written in red above.
Individuals can make their own decisions about how much capacity from both/either battery is sensibly worthwhile to add together, weight, space, cost and future reliability should be considered and is a personal choice.

Can I please ask anyone who feels a need to make a comment on the above to fully read what I've written before making a comment. If you disagree or don't understand then read it again and possibly again in case you've misunderstood.