Connection of new batteries to house bank - best method to avoid current surge?

[WindermereColvic]

Morning all,

I'm increasing the size of the house bank from 4*110Ah to 6*110Ah.

I'm conscious that the existing batteries will not be fully charged prior to making the connection, however the 2 new batteries will have been fully charged in the garage for a few days. Hence, when I connect them, I'm concerned that there will be a current surge until everything equalises out. Apart from a few sparks, is this likely to be high enough to damage anything?

Any thoughts on a connection method to avoid such a surge?

Thanks,

Matthew

 

[RichardS]

Ideally all the batteries should be charged.... And preferably the same type and age. Why not just turn off the main switch so the bank is not connected to anything then nothing can be damaged, by definition.

Richard

 

[pvb]

Apart from a few sparks, is this likely to be high enough to damage anything?

No, it won't be a problem.

With 6 batteries in parallel, it might be worth wiring the bank so that the main positive and negative connections go to opposite ends of the bank. This will help to even out the battery voltages - the benefit is very small, but probably still worth doing.

 

[scrambledegg]

No, it won't be a problem.

With 6 batteries in parallel, it might be worth wiring the bank so that the main positive and negative connections go to opposite ends of the bank. This will help to even out the battery voltages - the benefit is very small, but probably still worth doing.

I wouldn't be so sure. If the batteries to be added are at any significant difference in voltage to the existing bank, the energy transfer could be dramatic with hundreds of amps flowing through the short parallel connections. I don't know what effect that could have on the batteries, but it's probably not worth experimenting with it. I would be careful to equalise the new batterys' voltage to the old before connection, either by charging up the old, or by discharging the new.

 

[PaulRainbow]

Morning all,

I'm increasing the size of the house bank from 4*110Ah to 6*110Ah.

I'm conscious that the existing batteries will not be fully charged prior to making the connection, however the 2 new batteries will have been fully charged in the garage for a few days. Hence, when I connect them, I'm concerned that there will be a current surge until everything equalises out. Apart from a few sparks, is this likely to be high enough to damage anything?

Any thoughts on a connection method to avoid such a surge?

Thanks,

Matthew

• Turn the isolator off.
• Connect the new batteries.
• Turn the isolator on.

No harm shall befall you, the boat, the batteries or anything else in the vicinity. The Gods shall not strike you down, famine and pestilence shall not visit you.

IMO, the advice in post #3 is worth following.

Technically, all of the batteries should be the same type/size/make/age. In the real World i'm happy that my batteries are all the same size and type, i'm not bothered that one is slightly newer than the others.

 

[SteveSarabande]

I wouldn't be so sure. If the batteries to be added are at any significant difference in voltage to the existing bank, the energy transfer could be dramatic with hundreds of amps flowing through the short parallel connections. I don't know what effect that could have on the batteries, but it's probably not worth experimenting with it. I would be careful to equalise the new batterys' voltage to the old before connection, either by charging up the old, or by discharging the new.

It is only the same as connecting jump leads to a car with a flat battery.

 

[macd]

It is only the same as connecting jump leads to a car with a flat battery.

+1.
In addition, the same thing happens all the time on boats with an 1-2-both switch and only very rarely do they blow up. (And when they do, it's for other reasons.)

If you switch to 'both' with one full and one partially depleted bank, maybe something like 10A flow from the fully-charged bank, tapering rapidly to just a few amps as the load pulls down the voltage on the stronger side. I can't be the only one to have watched this with interest. It maybe different if the weaker battery is stone-flat, but that's not described as the case here.

As to wiring of the new batteries, this oft-linked page gives the skinny: http://www.smartgauge.co.uk/batt_con.html
It wouldn't be unusual for the OP to find that the four domestic batteries he already has are wired incorrectly.

 

[PaulRainbow]

It is only the same as connecting jump leads to a car with a flat battery.

Exactly !

 

[pvb]

I wouldn't be so sure. If the batteries to be added are at any significant difference in voltage to the existing bank, the energy transfer could be dramatic with hundreds of amps flowing through the short parallel connections. I don't know what effect that could have on the batteries, but it's probably not worth experimenting with it. I would be careful to equalise the new batterys' voltage to the old before connection, either by charging up the old, or by discharging the new.

As macd said, it's no different from using a 1-2-Both switch! Scaremongering doesn't help the OP.

 

[lw395]

It is only the same as connecting jump leads to a car with a flat battery.

I'm not sure that's a great recommendation!

A short burst of fairly high current will flow.
If you half-heartedly dap the connection on, there is possibility of sparking.
E.g. if you are placing an eye terminal then putting a bolt through it, the connection can get broken and spark while the current is still flowing.
Not great if you've just charged some of the batteries and there's a lot of hydrogen around.

Best to charge them all to the same voltage.
Then let any gas disperse.
If it is still a concern, connect initially with a lead with say a 1 ohm resistor in it, or even a test lamp lead. Let it equalise the volts then bolt up the connection.

 

[PaulRainbow]

I'm not sure that's a great recommendation!

A short burst of fairly high current will flow.
If you half-heartedly dap the connection on, there is possibility of sparking.
E.g. if you are placing an eye terminal then putting a bolt through it, the connection can get broken and spark while the current is still flowing.
Not great if you've just charged some of the batteries and there's a lot of hydrogen around.

Best to charge them all to the same voltage.
Then let any gas disperse.
If it is still a concern, connect initially with a lead with say a 1 ohm resistor in it, or even a test lamp lead. Let it equalise the volts then bolt up the connection.

Seriously ?

I mean seriously ?

 

[duncan99210]

Summer before last I added two batteries to the domestic bank mid season. The two new batteries were, presumably, charged to about 12.7v, whilst the existing one were probably at about 12.4-5v. I isolated the bank, fitted the new batteries and linked them into the system, turned domestic power back on and carried on as before. No bang, no weird results at all, simply a slight increase in the bank voltage to about 12.6v. It really isn't anything to worry about at all.

 

[Kinsale373]

I believe that connecting deep cycle batteries in paralled with very different charges will result in high current flow from one to another. These deep cycle batteries by design give high current and you have nothing in the circuit to limit the current.
I think your best plan is to run your engine for 15 mins to charge the existing bank prior to connecting in the new batteries. This should bring up the voltage pretty quickly and eliminate current transfer when you connect in the new units.
Or charge them up with a charger if you have shore power.
In theory you could let them equalise through a Resistance, say an incandescent light bulb for example. Insert this in the line between the two positives between the high charge battery and the low charge batteries. It may not light but the resistance will limit the current flow.
I don't think you need to do this though- just a thought, if you pre charge the existing bank you should be fine,

Kinsale 373

Kindale 373

 

[geem]

I think there are some people on this forum that have never used jump leads! Jump leads are designed to connect one nearly flat battery to one very charged battery. They spark. A lot. The current flows. A lot. Just connect the dam batterys........

 

[rogerthebodger]

Seriously ?

I mean seriously ?

Paul what these guys are forgetting in the internal resistance of the two banke of batteries that will limit the current flow between the batteries.

I have 2 banks with 6 105Ah batteries in each with the batteries in each bank being different ages. I very often see different voltages in each bank and when I connect the two banks together as recommended by pete I have I do see a current flow but only a fiw amps may be 10 to 15 is the battery banks has a much lover voltage that the other.

 

[Wandering Star]

I'd flog the boat to someone who's happy with 4 batteries and buy a new boat with 6 batteries already plumbed in by the builder - avoids all the possible risks and problems outlined here.

 

[macd]

I believe that connecting deep cycle batteries in paralled with very different charges will result in high current flow from one to another.

Belief is always trumped by measuring. See post #7.

These deep cycle batteries by design give high current...

Actually, starter batteries by design give high current (although that's not to say that deep-cycle batteries give negligible current). On the other the OP didn't actually mention deep cycle.

Regardless, both obey Ohm's Law. A difference in potential of a volt or less may give an initial spark, but it's not going to result in a huge flow of current.

 

[lw395]

One of the things I've worked on is comms, with big standby battery banks.
They have procedures for connecting new cells for a reason.
I tend to take the sparks and hydrogen risk fairly cautiously, having seen the mess a mate of mine made by blowing up a car battery.
An enclosed boat is not the greatest place to be fiddling with batteries.

 

[geem]

One of the things I've worked on is comms, with big standby battery banks.
They have procedures for connecting new cells for a reason.
I tend to take the sparks and hydrogen risk fairly cautiously, having seen the mess a mate of mine made by blowing up a car battery.
An enclosed boat is not the greatest place to be fiddling with batteries.

Batteries explode for other reasons. Connecting healthy batteries together is not one of them.
Nobody is suggesting doing surgery on the batteries. Simply connecting them together
In comms installations batteries are connected in banks at high voltages such as 220v or higher. Of course there will be strict protocol due to risk of electric shock, etc. Here we are talking about a very small battery bank in comparison at 12v. Where is the problem?

 

[PaulRainbow]

During the Winter i was jump starting several cars a day, some with batteries slightly flat, some as flat as a witches tit. My van is fitted with two heavy duty starter batteries, a high output alternator and has a set of very heavy jump leads wired into the batteries. I connect the leads to a variety of battery sizes and have never, ever, had an issue.

Sometimes the batteries are very flat, there are no big sparks, no smoking jump leads, no death and destruction.

Mentions of hydrogen are red herrings. There will be no hydrogen present (in levels capable of causing an explosion), given the description by the OP. His boat batteries will not be giving off hydrogen, why would they ? The new batteries have been charged away from the boat, so again, no hydrogen.

Sparks will be irrelevant, due to the lack of hydrogen. But, sparks will be minimal or virtually non-existent anyway. Normal practice is to connect all the positive cables first, so no possibility of sparks when doing so (as no neg connection). Last thing is to connect the negative cable, possibly a tiny spark here, but of no consequence.