linking batteries

[pete-filldir]

hi

If you have a 2V cell @240ah and you connect six together to make a 12 V battery would that then make a battery of 1440ah, or would it stay at 240ah?

[frotter]

Sorry - no.....  12v 120ah

 

[billi]

frotter was first

no remains 120ah

multiplied by 12volt then   you have theoretecally then 1440 watt power at 220 volt

so advicable only 750 watt at 220 volt before recharge


sorry wrong reading ...  you have 240 ah  at 12 volt   then its  twice ...


sorry 

billi

[billi]

again another try

240 ah at 2 volts  wired to a 12 volt battery (6 cells)  will be a 240 ah battery at 12 volt



  puu  hope iam right now


regards billi

[pete-filldir]

OK thanks for the replies

another question, can you connect batteries with different ah, eg 85ah and 110 ah?

cheers

Pete

[billi]

not a good idea as far as i know

[Cliff top]

If they are the same type and are both full and of fairly close Ah then there shouldn't be a problem.

If one isn't full then a huge current will flow into the empty one, possibly stressing the full one too much and damaging the plates.

Once connected they will fill and empty together and stay balanced

[martin]

actually, they won't! This is the reason manufacturers always tell you not to mix even batteries of the same capacity but of a different age.......to mix batteries of both different age and capacity is asking for the early demise of one or both!
About the only way I can think of to safely use the two is to use a split-charge device, and then take any loads from one battery or another in turn - this will incur losses, but ensure the batteries last!
http://tinyurl.com/2t72wj
Chapter and verse on doing it right! http://www.smartgauge.co.uk/batt_con.html
and here! http://www.smartgauge.co.uk/technical1.html

[northern installer]

To over simplify,think of the batteries as resistors;each battery has its own internal resistance value,if you connect two or more in parallel,current will flow in the path that offers least resistance.It is quite likely that one battery will lose charge to its neighbour ,unless as Martin says,you connect via a split charge diode.Sudden and violent discharge in the event of an internal short can be prevented by providing a suitably sized fuse in one side of each interconnector,better than having hot sulphuric acid dripping from the battery house ceiling

[pete-filldir]

thanks for that,
 glad I asked first!!!

[martin]

I have been heard to threaten the setting up of the National Society for the Prevention of Cruelty to Batteries!
I see the poor things misused and abused, even by those who should know better - I think the problem is that they're seen as the least "sexy" part of a system, and often people try to cut corners when buying/setting up their battery bank. Add to that the fact that people are accustomed to "chuckaway" batteries, and try to treat rechargeables the same - knackering them right out before recharging etc.................
I think batteries are much maligned - they can give you much-needed independence from centralised power supplies, and are well over 90% recyclable using relatively low-tech methods (I'm a fan!)
Just choose the right ones for the job, wire them correctly, and design the system to suit their characteristics...........

[northern installer]

well said Martin,yes there is a definate place for batteries,even if they are less than perfect devices;but(old Auntie time again) lets just examine some of the hazards for a moment:there seems to be a sort of'its only 12 volts,cant harm you' attitude to batteries,but in reality,its quite the reverse.A simple car battery can produce a fault current in excess of  1000 amps.The gases given off on charge or discharge are explosive.The electrolyte within is sulphuric acid. Put these three together,add that spanner you put in your top pocket falls out as you bend over the battery bank;it welds itself to the terminals,boiling the cells;the gases ignite from the severe sparking......oh dear         Simple and pretty obvious precautions:ALWAYS wear goggles and gloves when handling or working near batteries,including that quick level check,dont wear rings or medallions(!);put the battery bank in a suitable shed or enclosure,not the one you keep all the spades and lawnmower in.When working near the batteries,put a mat,or a strong cloth over the terminals,better still,a purpose made plywood cover on each one;Wiring- use the correct terminals for the battery posts,poor connections cause hotspots,there must be sufficient surface area for the energy involved.Use the correct crimp or solder terminals on all leads.Put a suitable size fuse in circuit as near to the battery as possible;make it hrc,then the arc is enclosed and wont ignite gases (a good source of hrc fuses and carriers is from scrap mains switchgear) put a fuse between parallel batteries.Ensure adequate ventilation at high level to deal with the hydrogen.Make sure outgoing cables are protected by suitable fuses,No good going to all this trouble then burning the house down is it! have fun

[Bob]

Generally I agree with everything Martin said but

to mix batteries of both different age and capacity is asking for the early demise of one or both! Roll Eyes

has to be qualified.  There is often confusion over the difference between "battery" and "cell".  A cell can be described as the smallest unit available of a particular electro-chemical configuration.  A Battery is a collection of linked cells.  What is normally termed a "AAA battery" is in fact a single cell.  The thing under the bonnet of your car is a battery, it's 6 lead-acid cells in series.

The Ah rating of a lead-acid battery is determined by the number of plates in any of its individual cells.  The voltage is determined by the number of cells you have linked.

I have very successfully mixed cells of identical type but different ages to form new batteries and combined those batteries in parallel configurations.  In parallel you increase the Ah rating while keeping the voltage the same.

If you want to mix batteries of different voltages then a DC-DC converter and sensing relay are normally needed.

On northern installers safety point I love the blade fuses that normally come in threes for high power 3ph supply systems.