Wiring 8 batteries in parallel

[MHS]

I am wanting some advice on how best to connect 8 110Ah batteries in parallel.

 

I read carefully the web-page mentioned on the forum: http://www.smartgaug...k/batt_con.html

 

I had hoped to use method 2 but i assume that the imbalance in charging would be greater the more batteries were added?

 

I am hoping someone can clarify how method 4 would best work with 8 batteries.

 

We currently have 4 (newish) batteries which I do not think are wired as they should be, and want to sort it all out when adding another 4 to the bank. I'm onboard tomorrow with my newly purchased clamp meter as per members advice from my previous thread.

 

I would be very grateful for some help.

 

Mike

[by'eck]

I think you have the wrong link to Smartgauge site so have added it here.

 

With eight batteries I would be tempted to use method 4 but with four pairs of batteries instead of the two pairs shown.

[Ex- Member]

With method 2 you would take off each end of your batteries a couple of batteries in each end. So wire Pos cable to bat 3 and Neg on bat 6.

 

We went with diagram 3, IMO a far superior way of doing this. You end up with 2 terminals which everything connects to and can be well isolated from the battery terminals. The golden rule being that once you've connected your battery bank, nothing else should be connected directly to the battery bank, with the exception in some cases of censoring equipment.

 

 

It's also not good practice to mix new batteries with old, the older batteries could well have a detrimental effect on the newer ones depending on their condition.

[nicknorman]

You will note a comment near the end of that page that says "Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from somewhere down the chain instead of from the end batteries." So maybe put the connections on batteries 3 and 6.

 

However I maintain that this whole issue is over-egged provided adequate interconnecting cables are used. We had 4 x 110A linked with 70mm² cable with the connections all at one end. Using the clampmeter I couldn't detect any difference between the charging and discharging currents (even at high values such as 200A) of the batteries at one end vs the other, and neither could I detect any difference in the voltages across the batteries using a digital voltmeter. I am sure there were differences, but they were so tiny as to be completely inconsequential.

 

More important IMO is to ensure that adequate cabling is used and the connections are all maintained in good condition. If say 25mm cable is used and the connections are corroded or badly crimped then yes I can see that having equal current paths is necessary, but surely it is better simply to have and maintain good interconnects?

[Ex- Member]

You will note a comment near the end of that page that says "Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from somewhere down the chain instead of from the end batteries." So maybe put the connections on batteries 3 and 6.

 

However I maintain that this whole issue is over-egged provided adequate interconnecting cables are used. We had 4 x 110A linked with 70mm² cable with the connections all at one end. Using the clampmeter I couldn't detect any difference between the charging and discharging currents (even at high values such as 200A) of the batteries at one end vs the other, and neither could I detect any difference in the voltages across the batteries using a digital voltmeter. I am sure there were differences, but they were so tiny as to be completely inconsequential.

 

More important IMO is to ensure that adequate cabling is used and the connections are all maintained in good condition. If say 25mm cable is used and the connections are corroded or badly crimped then yes I can see that having equal current paths is necessary, but surely it is better simply to have and maintain good interconnects?

 

I thought the idea of wiring batteries in this manner was to ensure equal charging through the bank and keeping each individual battery basically equal to each other.

 

I recall when we initially had 6 batteries and poz & neg leads connected to one end that some individual batteries failed over time and one even exploded, that being the one the leads were connected to. I'm sure the way they were wired was the cause of such problems. I wonder though on a smaller bank of 4 batteries as you mention the effects would be less.

Edited August 2, 2014 by Julynian

[nicknorman]

I thought the idea of wiring batteries in this manner was to ensure equal charging through the bank and keeping each individual battery basically equal to each other.

 

I recall when we initially had 6 batteries and poz & neg leads connected to one end that some individual batteries failed over time and one even exploded, that being the one the leads were connected to. I'm sure the way they were wired was the cause of such problems. I wonder though on a smaller bank of 4 batteries as you mention the effects would be less.

It is, however IMO it is mostly a "theoretical" issue rather than a practical one, provided as I said, the interconnects are adequate and in good condition. For it to be an issue, surely there would have to be different currents flowing in and out, different voltages on the each battery. But that was something I was completely unable to find on our "incorrectly" wired bank. Clearly the effect is less with 4 batteries but I think it will be minimal on an 8-bank as well.

 

I am not saying "just stuff the cables on one end" - if one is going to wire it up, one might as well comply with some diagonality. I am just saying "don't stress about it, it is more important to ensure that what interconnects you have are in good condition and adequately sized".

[Ex- Member]

It is, however IMO it is mostly a "theoretical" issue rather than a practical one, provided as I said, the interconnects are adequate and in good condition. For it to be an issue, surely there would have to be different currents flowing in and out, different voltages on the each battery. But that was something I was completely unable to find on our "incorrectly" wired bank. Clearly the effect is less with 4 batteries but I think it will be minimal on an 8-bank as well.

 

I am not saying "just stuff the cables on one end" - if one is going to wire it up, one might as well comply with some diagonality. I am just saying "don't stress about it, it is more important to ensure that what interconnects you have are in good condition and adequately sized".

 

Yeah I see what you mean and as in method 2 it's no more difficult than doing it incorrectly As we did initially LOL

 

I chose method 3 as we were fitting a completely new installation, (with your assistance you might recall) but correcting an existing bank, again method 2 makes good sense and you probably wouldn't need new cable & connectors.

[MHS]

With method 2 you would take off each end of your batteries a couple of batteries in each end. So wire Pos cable to bat 3 and Neg on bat 6.

 

It's also not good practice to mix new batteries with old, the older batteries could well have a detrimental effect on the newer ones depending on their condition.

I also thought connections to 3 and 6 may be the way to go.

The batteries are virtually new as our boat only went in the water 7 weeks ago.

 

I am planning to use 40mm interconnecting cables.

[Ssscrudddy]

I would have thought method 3 would be best

[Ex- Member]

I also thought connections to 3 and 6 may be the way to go.

The batteries are virtually new as our boat only went in the water 7 weeks ago.

 

I am planning to use 40mm interconnecting cables.

 

Yeah 3 and 6 definitely, 40mm would be amply as well. If existing batteries are that new there shouldn't think it would be a problem.

[MtB]

EIGHT batteries????

 

I manage fine with two!

 

 

(Hope that helps )

 

MtB

[nicknorman]

I also thought connections to 3 and 6 may be the way to go.

The batteries are virtually new as our boat only went in the water 7 weeks ago.

 

I am planning to use 40mm interconnecting cables.

Before being able to know whether 40mm is adequate we need to know the max current. What size is the alternator? If there is an inverter fitted, what power rating? We have 70mm with a 175A alternator and 2.5kw inverter (max current a little over 200A) which seems about right. If you have something similar I would be inclined to go for 70mm since the additional cost is minimal, bearing in mind the short lengths. Just make sure you use a proper crimp tool, and protect the bit where insulation ends and crimp begins, with eg adhesive heat shrink or at least tape. I put a bit of Vaseline into this gap after crimping and before heat-shrinking, although with our Trad stern the batteries don't get damp.

 

Even if you don't have a large inverter, you might as well "future proof" the installation in case you want to add a large inverter at a later stage.

Edited August 2, 2014 by nicknorman

[MHS]

We have a Beta marine with a 175 amp alternator and a 3kw Victron inverter.

[nicknorman]

We have a Beta marine with a 175 amp alternator and a 3kw Victron inverter.

I would definitely use 70mm interconnects then.

 

Ed: Just to explain, there are 2 considerations when selecting cable. One is the amount of heat generated / temperature rise of the cables, the other is voltage drop. At 12v, any voltage drop is much more significant than at mains voltages and can often be the determining factor in cable selection on boats.

 

So a 3kw inverter running flat out will be taking around 275A. With the wiring as discussed (taking the connections off batteries 3 and 6) that means the interconnect from say battery 4 to the takeoff point on battery 3 will be passing 5/8 x 275A = 172A. More if the battery voltage dips under load to below 12v. That represents a 50 deg C temperature rise in a 40mm cable (if sustained). With ordinary PVC sheathed cable typically being rated only up to 80deg C that means an ambient temperature of 30 deg C is the maximum to avoid exceeding the temperature rating of the cable.

 

OK I admit that sustained 3kw is unlikely, but it is possible.

 

By contrast 70mm cable has both lower resistance and more external area to dissipate the heat, and will run a good bit cooler.

 

In this case, because of the short lengths of cable, probably voltage drop is less of an issue but on the other hand the whole issue of battery balancing starts to become significant with relatively small voltage drops.

Edited August 2, 2014 by nicknorman

[Robbo]

http://flic.kr/p/oAkhPX

The image above is like smileypetes idea that's on gibbos site, but I've extended it to 8 batteries.

 

On iPhone so excuse the poor drawing!

 

 

Edit to add you can take the feeds from the middle batteries if wished instead of the end ones. Just make all the links the same size and length cable for each type of link.

Edited August 2, 2014 by Robbo

[nicknorman]

http://flic.kr/p/oAkhPX

 

The link above is like smileypetes idea that's on gibbos site, but I've extended it to 8 batteries.

 

On iPhone so excuse the link and poor drawing.

Fine, but phenomenally complicated, and unnecessary so IMO.

[Robbo]

Fine, but phenomenally complicated, and unnecessary so IMO.

Not really, it uses the same amount of links, just that some are longer and connected to make it balanced.

[mango]

Fine, but phenomenally complicated, and unnecessary so IMO.

I agree. The complicated wiring would be best option if you are trying to get the best out of 25mm² cable, which some people insist on using. Using the right cable for the job and making sure the connections are sound is better, as you say.

 

It's really important to make sure that connections stay good when batteries are replaced and to inspect the battery post connections regularly for signs of corrosion.

[MHS]

I would definitely use 70mm interconnects then.

 

Ed: Just to explain, there are 2 considerations when selecting cable. One is the amount of heat generated / temperature rise of the cables, the other is voltage drop. At 12v, any voltage drop is much more significant than at mains voltages and can often be the determining factor in cable selection on boats.

 

So a 3kw inverter running flat out will be taking around 275A. With the wiring as discussed (taking the connections off batteries 3 and 6) that means the interconnect from say battery 4 to the takeoff point on battery 3 will be passing 5/8 x 275A = 172A. More if the battery voltage dips under load to below 12v. That represents a 50 deg C temperature rise in a 40mm cable (if sustained). With ordinary PVC sheathed cable typically being rated only up to 80deg C that means an ambient temperature of 30 deg C is the maximum to avoid exceeding the temperature rating of the cable.

 

OK I admit that sustained 3kw is unlikely, but it is possible.

 

By contrast 70mm cable has both lower resistance and more external area to dissipate the heat, and will run a good bit cooler.

 

In this case, because of the short lengths of cable, probably voltage drop is less of an issue but on the other hand the whole issue of battery balancing starts to become significant with relatively small voltage drops.

Will have a look tomorrow to see what size is currently fitted.

Slightly confused that if you recommend using 70mm interconnecting cables, that they wouldn't be fitted already to every 4 battery set up running a 3kw inverter.

[PhilR]

EIGHT batteries????

 

I manage fine with two!

 

 

(Hope that helps )

 

MtB

 

Me too. But mine is a leisure boat and I don't need to power lots of electrical contraptions that live-aboards might see as more essential.

I used to have 3 batteries, but for my needs, decided that 2 good batteries would be cheaper and more easily maintained. So far the 2 batteries are in great shape and it is easy to keep them at or near fully charged

[nicknorman]

Will have a look tomorrow to see what size is currently fitted.

Slightly confused that if you recommend using 70mm interconnecting cables, that they wouldn't be fitted already to every 4 battery set up running a 3kw inverter.

Well they are certainly fitted to our 4-battery setup! With these things there is no "right answer" - yes you can probably get away with 40mm but for the very slight cost increase why not reduce power wastage through heating, reduce the chances of a cable overheat, and reduce differential voltage drop between batteries. It just seems to make sense to me!

[MHS]

 

Me too. But mine is a leisure boat and I don't need to power lots of electrical contraptions that live-aboards might see as more essential.

I used to have 3 batteries, but for my needs, decided that 2 good batteries would be cheaper and more easily maintained. So far the 2 batteries are in great shape and it is easy to keep them at or near fully charged

Teenage daughters / wife seem to like having hair driers and straightners. (Not needed for my own shiny head) Also a tv a little larger than 14". None of them individually draw much power, but it all adds up.

[Robbo]

 

Me too. But mine is a leisure boat and I don't need to power lots of electrical contraptions that live-aboards might see as more essential.

I used to have 3 batteries, but for my needs, decided that 2 good batteries would be cheaper and more easily maintained. So far the 2 batteries are in great shape and it is easy to keep them at or near fully charged

More capacity means longer periods between charging. So we may have the same power usage, but I only need to charge once a week where you may need to do once a day. Not a big issue for leisure users, but for liveaboards it's handy.

[nicknorman]

Not really, it uses the same amount of links, just that some are longer and connected to make it balanced.

True same number of links but as you say, significantly more cable required. Also having 4 cables going onto 1 terminal can create problems of the physical space and bolt length needed to do that.

[mango]

 

Me too. But mine is a leisure boat and I don't need to power lots of electrical contraptions that live-aboards might see as more essential.

I used to have 3 batteries, but for my needs, decided that 2 good batteries would be cheaper and more easily maintained. So far the 2 batteries are in great shape and it is easy to keep them at or near fully charged

That suggests that gas fridges are making a comeback.