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[IanPal]

Hi guys, Just a quick question, Would it be ok to link four 12v 7Ah batteries to give me 28Ah?


When I've linked two before I've used an Action P103 Parallel board, but cant do this with four can I?


Any help would be great before I see it all go up in smoke 


Ian

[Bob K]

Yes, but you need 4 identical 12V batteries, preferably new, as one battery performing slightly less could have adverse effects.  You may try a strip of Faston PCB strip terminals, soldered onto a copper strip, 5 way each for +ve and -ve.  That way you have identical wire sizes to a common point, then a thicker wire to where you draw your load.    Coat the strips (except for the 1/4" tabs of course) with an insulator such as liquid plastic tape or insulating resin.

That is one heck of a current draw !!!

[inertia]

Maximum current 15A per P103 i.e. 30A total. Minimum 16AWG multi-strand cable recommended.
DM

[Bob K]

I doff my cap to Dave.  A true professional 

[IanPal]

Thanks Bob and Dave, I'll order a couple of boards on Monday then, I need some more wire anyway.


Thanks again
Ian

[Southern Sailor]

Hi Ian. I am not a clever sparky so the question may be stupid, but why can't you just connect them together in parallel?  Cheers.  Brian

[IanPal]

Hi Brian, You can just wire them in parallel, and it would work giving me 28Ah,


But if one had more power in it than any of the others you run the risk of the higher charged battery back charging into the others.
This may explain it a bit better than I can...


http://www.componentshop.co.uk/p103-parallel-power-board.html


The thing is though, I've been thinking about it and on the component shop site they have a battery 12v 28Ah for £30, so think I'll go that route, same outcome for the same money as two boards, but a lot less wiring.


Ian

[Southern Sailor]

I understand.  The parallel power boards prevent the one charging the other by the action of the diodes on the boards.  Clever these guys. Is the 12V 28AH battery not bigger than the 7AH?


Thanks and cheers. Brian.

[Jerry C]

I wonder if this reverse current problem is more theoretical than a practical reality. On my narrow boat I have 4 12 v 110 amp/hr batteries wired in parallel for the domestic (leisure) supply. They are not fitted with diodes. On most of the tugs I've worked on we had 20 or so 12v 300 amp/hr deep cycle lead acid batteries wired in series and parallel to give 24 volts, again without diodes. This is standard practise in both situations. However battery banks are monitored weekly for condition and specific gravity. This is not possible with sealed lead acid batteries and so diodes may be the way to go. On my model of The voith tug Parat I have 2 12v 7 amp/hr batteries wired in parallel driving one esc controlling 2 motors in parallel. The rig is 6 years old and has never given any cause for concern, without diodes.
Jerry.

[IanPal]

I can see where your coming from Jerry, In fact thinking about it being an ex lorry driver, all the old trucks I had were two 12v wired in series to give you 24v, again no diodes, and I never experienced any problems.


Does make you think I must admit.


Brian, yes the 28Ah is a lot bigger in size than one 7Ah, but four of the smaller ones sat side by side will be much the same as the bigger one, But looking on component shop site, the bigger one is around 1kg less.


Ian

[inertia]

On my model of The voith tug Parat I have 2 12v 7 amp/hr batteries wired in parallel driving one esc controlling 2 motors in parallel. The rig is 6 years old and has never given any cause for concern, without diodes.
Jerry.

Presumably they were bought for that purpose and fitted together, and have remained in that model all of the time? They're charged at the same time, for the same amount of time, and they supply the same load. You also sound like the sort of guy who treats his equipment properly. No problem.
P103 wasn't intended for perfect situations; neither is its use solely for lead-acid batteries. It's an insurance against wildly-differing batteries, especially NiMH ones, which can back-charge from one pack to the other with some violence. The dual Schottky diode on P103 is a safety precaution - just as is fitting a fuse to a circuit.

Ian
Two 12v batteries wired in series does not pose the risk that parallel wiring does.

DM

[IanPal]

Thank you Dave, I didn't know that, I just took it for granted that which ever way they were wired it would be the same.


You learn something everyday 


Ian

[inertia]

Just for interest/information:
In the top diagram the batteries are connected in series, with the total voltage V3 being the sum of the two batteries V1 and V2. There is no circuit so there can be no current until a load is connected. You can use any value for V1 and V2; they don't have to be the same.
The second one is very different. The act of connecting the two batteries together like this forms a closed circuit (shown by the orange ellipse). If the voltage in either battery is greater than that in the other then a current will flow, as one battery tries to charge up the other one. In extreme circumstances these currents can be very high - enough to cause damage to the batteries and anything else in range! You can see therefore that the two batteries must be as near identical as possible and that the total voltage V3 will be the same as that in each battery. Also beware that if you use different capacity batteries then one will become discharged before the other and back-charging could occur.

[malcolmfrary]

It isn't a theoretical problem - if the two batteries are charged differently, when connected, the charge will even out, usually very rapidly, causing the electrolyte to fizz and bubble a bit.  On open or unsealed batteries, the water can be replaced.  On sealed ones, it is quite possible that venting will happen which does shorten the battery life since the lost electrolyte can not be replaced.  If the evening-out current gets high enough, it could also warp the plates. 
It is often forgotten, in this modern age of lithium batteries, that SLA batteries do have a very low internal resistance, and are quite capable of delivering a heck of a high current, just not for long.  And, of course, fr a given amount of energy stored, they do weigh a lot more.
Batteries that are similarly charged (i.e. as near identical voltage as a digital meter can show you), will not give any trouble when being connected, and as long as they remain connected, when being used, charged and discharged as one unit, they will effectively be one bigger battery.



Noted Daves reply while I was keyboard pecking.

[Jerry C]

All understood DM. Re Parat you're quite correct. I'm only amazed the SLAs have lasted so long. I use a charger I made from one obtained from a scrap Furono radar used to maintain 2 x 12v 7amp/hr batts for the system menus. I just put it in a box with ammeter, voltmeter discharge lamp and various leads for different purposes ie. boat batts and motorbike in winter. On the tugs we use 24v for most navigation equipment and control systems. We generate 440v 3ph 50hz and transform to 240v for domestic etc. and then down to 24v DC for all the important stuff. We run off the main switchboard normally and the batteries ar purely back up. On a new build (Chinese), the batteries were not secured correctly, missed by Lloyds  surveyors, and in a hurricane 240miles off Finisterre they shifted and the output wires chaifed on a metal bar shorting out the lot. The resultant fire was extinguished by me and the C/Eng. when we opened the locker the next day it looked like a model of Chernobl. 3 batteries were saved and a jury rig sorted out which got us into Coruña after 2days of hell. Repairs took 5 days. For anyone interested she was the Svitzer Gelliswick on passage from China to Milford Haven Via the Cape.
Jerry.

[inertia]

Jerry
I don't think a humble P103 would have saved you from that!
Like almost every ACTion unit, it was introduced at the suggestion of a customer who'd identified the risk in his model. I originally used to fit two separate diodes to a modified P95 PCB but clever old Dr Wombat found the dual Schottky hiding in Farnell's catalogue and did a proper circuit board for it. It does have a pretty low maximum voltage and current (15v and 15A total) but there are higher-value diodes available in the same physical package (TO220) which can be used in conjunction with a larger heat-sink.
Your mention of fitting the charger into a box etc made me smile; many of the folk I know wouldn't know what a meter was, let alone how to use one. As for a discharge lamp...........    
We try. Meanwhile for those who haven't yet discovered it, here's a whole mess of useful stuff in the same vein as the posting above  http://www.modelboats.co.uk/news/article/but-i-don-t-understand-electronics/18054
Dave M

[Calimero]

Hi guys, Just a quick question, Would it be ok to link four 12v 7Ah batteries to give me 28Ah?

Just for my information, what kind of boat are you building that requires such capacity (and could host that much weight) ?

At some point, SLA batteries are more of a problem rather than a solution.

[grendel]

in my caravan I had 20 6v 10AH batteries wired in 10 sets parallelled, ie two batteries in series to give 12v, then 10 of these pairs wired in parallel, all were identical batteries. the problem comes when 1 battery fails, it can drag down the whole of the set with it, after several years this set was dismantled, and only 7 or 8 batteries remained viable, the rest are just scrap ballast.
the other option is to seperate all the batteries when charging
Grendel

[NFMike]

The thing is though, I've been thinking about it and on the component shop site they have a battery 12v 28Ah for £30, so think I'll go that route, same outcome for the same money as two boards, but a lot less wiring.

   Often doing it the right way is no more expensive than concocting a dog's dinner.

[malcolmfrary]

A few spread around can be a lot easier to site than one great big one as against the single big 'un being easier to wire up.  Swings and roundabouts, executive coin flip.

[IanPal]

Just for my information, what kind of boat are you building that requires such capacity (and could host that much weight) ?

At some point, SLA batteries are more of a problem rather than a solution.

Hi Calimero, The boat is an oil tanker, bought second hand. It just needs some TLC.


She is just short of 6ft long, beam is 12 inches and height is approx 17 inches to top of superstructure.  (still trying to work out it's scale) It doesn't really require such capacity I know. But who ever built her has made the battery tray that a ride on lawn mower battery will fit in easily.


I just tried to add a picture but it's saying pic is to large.
[/size]
[/size]The motor that's in her is a car blower motor.
[/size]I just ordered a bow thruster for it, which came out more expensive than I paid for the whole boat


I'll have a look later to see if I can reduce the size of the pictures.


Ian




   

[IanPal]

Opps don't know what happened there, What I said was the motor that's fitted is a car blower motor.


I ordered a bow thruster last week, as there hasn't been one fitted, and with it's size I have a strong feeling its turning circle will require the best part of my local pond. (although that would be a bit to scale I guess) 


I Just tried adding some pictures but they are too large, will reduce the size later if I can.


Ian

[Calimero]

If you don't need all that capacity do you think it's worth the cost and trouble to get all those batteries ?

You could use maybe only a couple of them and use scrap metal as ballast instead ?

[sparkey]

  Years ago I was given a load of ex fire alarm batteries(12v 7ah) which were in different stages of  their life,at that time I had an old tug which I was restoring and intend to use 2 of these batteries in parallel,as Malcolm said when 2 batteries are in parallel they become 1,so to overcome this problem I discharged both with a car headlight bulb till nearly flat then wired in parallel and charged as 1 battery,I used this boat for many years and sold it on to another boater who used it further years,not bad for batteries that were going to be thrown out  when they updated the fire alarm system,I also got some 12v 17.2ah batteries which sit in my tool box to charge other batteries at the lake side,I use a Turnigy accucell 6 charger for this purpose,they must be 20 years old how and still doing the job,Ray.