Assembling battery bank using different capacity units

Thunderchild69 · Veteran Member

Assembling battery bank using different capacity units

Thunderchild69 · Veteran Member

I currently have 2 X 100 Amp AGM batteries and have acquired 2 X 120 Amp AGM batteries that I wish to gang together to increase my storage reserve. Can anyone advise if there are any problems ganging slightly different capacity units together in the same bank? I intended hooking them up using "Method 4" here: http://www.smartgauge.co.uk/batt_con.html with two banks of two (1 X 100 Amp and 1 X 120 Amp in each of two banks).

Thanks in advance confuse

TC69

brian · Guru

There's no problem paralleling batteries of differing capacities provided they are of similar chemistry and age.

Even if of different chemistries, ie an AGM and a flooded in parallel, it is not a real concern if the lower charging voltage of the pair is used and equalization is disabled.

If their ages are very different, then their internal resistances will also be different, and this will cause the newer battery to work harder, so they will most likely end up dying at roughly the same time anyway.

That Smartgauge set up would be fine, but I'm not sure what you mean by "two banks of two".

If the 2 banks are separated from each other, then you would be better served connecting 2 batteries in parallel with the +ve and -ve leads coming from opposite corners and then connecting those wires to the same configuration on the other bank, otherwise you will be running too many wires.

An RV generally doesn't have the high loads that demand the interconnections that they show unless it is a simple matter to achieve. You would lose more in voltage drop by running the extra interconnections between the 2 separate banks than you would gain in efficiency.

Again, the usual RV loading (normally less than 100 amps and for short enough periods that the batteries will soon re-equalize when the load is removed) does not really require any special configuration of the interconnecting wiring.

Those Smartgauge diagrams are mainly intended for starting large marine engines where instantaneous currents of a thousand amps or more may be required. If you have a look at method 2, you will note that those connections would be much easier to achieve for a split bank, and the differences in their example at 100 amps load is just 3.5 amps, that's not a problem at all in this case.

brian · Guru

I would suggest that your biggest voltage drop concern in this installation will be the master switch. It can also cause problems if you inadvertently have it in any position other than BOTH, as you will then be drawing all of your current, and possibly charging, only one bank. My suggestion would be to fully parallel both banks and either get rid of the master switch or just use one side of it. You can co-join terminals 1 & 2 at the switch to ensure that you don't accidentally isolate the bank by switching to the wrong number.

The following information is assuming that you do the above, as otherwise your individual load currents and voltage drops will be higher if only one bank is supplying your inverter, and the recommended cable sizes will need to be increased.

Even though the inverter is rated at 2500 watts, it's unlikely that you will get close to that unless you run 2 of those appliances at once, which I would suggest you don't do.

The coffee machine would, I expect draw around 1100w (about 90 amps) for a short period, the toaster about the same, and the microwave, assuming the 1100w is the power consumption figure and not the cooking power, will be about the same again.

Add around 2 amps for the quiescent current and efficiency losses of the inverter, and you will be drawing a tad under 100 amps for as long as the appliance is in its high state. For a toaster, that's all the time it is cooking. For the coffee machine it is only whilst heating but will drop to a much lower figure in the keep warm mode. For the microwave, it is only whilst the Magnetron is in operation, which will be about 60 - 80% % of the time on high, plus a little for the electronics and the turntable.

So, worst case would be a draw of around 100 amps

Split this between two banks and you have just 50 amps from each bank. Not a heavy load.

Provided you use a decent sized cable, minimum 9AWG or 6mm², between the batteries, and minimum 8AWG or 8mm² between the banks then you would have no problems keeping them charging and discharging equally.

Depending on the distance of the inverter from the batteries, and you should keep it as close as possible, you would use a cable of minimum 4AWG or 20mm². If the run is short, make sure the wires are well protected and don't include any fusing in the inverter circuit.

Make sure that all connections are clean, well terminated and tight, hire a crimper if necessary, it's more reliable than soldering.

Thunderchild69 · Veteran Member

Again Brian - thanks so much for sound advice. Had already covered all of your suggestions when I originally wired in the solar panels and inverter except the master switch. It is always selected to both, but I like your suggestion to use one pole and "Murphy proof" it when I add the additionally batteries.

Thanks again

TC69

oldbobsbus · Guru

Hi Brian,
As you as giving very sound advice I wonder if you could run your experienced eye over this set up and tell me if I should make any changes..
It is running a 320ltr Fridge 24/7 along with anything else we turn on from time to time..
The whole system is 24v as you can see and all the battery interconnecting cables are 200a welding cable with soldered ends.

The solar panels are putting out 32v and most day the batteries are at 25v at 7.00am and 27v at 5.00pm depending on light..

brian · Guru

oldbobsbus wrote:
Hi Brian,
As you as giving very sound advice I wonder if you could run your experienced eye over this set up and tell me if I should make any changes..
It is running a 320ltr Fridge 24/7 along with anything else we turn on from time to time..
The whole system is 24v as you can see and all the battery interconnecting cables are 200a welding cable with soldered ends.
The solar panels are putting out 32v and most day the batteries are at 25v at 7.00am and 27v at 5.00pm depending on light..

Looks good to me, provided the soldering is well done. It's generally more reliable to crimp large cables due to the heat required to properly make the solder suffuse throughout the wire strands, this can be especially difficult to do with the very large number of strands in a welding cable.

If you were to be really pedantic you would have the inverter wires on the same terminals as the solar wires to ensure equal load on both battery pairs, but it's pretty academic with the cable size and small distance that you are showing.

Be a bit careful rating cables by their current carrying capacity, as that is very dependent on circuit length, heat dissipation and duty cycle.

For example:- An unenclosed 35mm² welding cable is rated at 205 amps 100% duty cycle, a 25mm² one is rated at 257 amps but at just 25% duty cycle, and only 165 amps at 100%, and that's without taking length of circuit into consideration. Run 2 of those 35mm² wires together on a flat surface and their capacity is reduced to just 140 amps

So.. stating the ampacity of a cable without also defining the duty cycle, circuit length and installation method can be misleading.

Also, don't be fooled by an unrested voltage reading on a battery. Surface charge very rapidly brings even a quite flat battery up to a reading that can be misconstrued as being fully charged, and likewise, surface discharge can give the opposite effect. This surface charge effect is why an alternator alone doesn't do a real good job of charging a battery.

I would expect the 7am reading to more indicative of true battery Soc than the 5pm one.

The only other suggestion would be to fuse the +ve wire from the battery to the solar controller.

Hope this helps.

oldbobsbus · Guru

Thanks Brian,
I gave the 5.00pm charge as at that hour the panels aren't putting much in maybe .4amp and when I am using the bus I am able to check the battery voltage of a night as I have a voltage gauge inside the bus and it usually holds around the 25.5v mark until I give the pillow a nudge.

The solar controller has built in 40a fuses on both the incoming +ve circuit and also the outgoing +ve circuit.and also supplies a set of spare fuses..

I fully understand about soldering heavy cable as I used to make welding cables and always soldered them so I am able to get the job done properly..

brian · Guru

oldbobsbus wrote:
Thanks Brian,
I gave the 5.00pm charge as at that hour the panels aren't putting much in maybe .4amp and when I am using the bus I am able to check the battery voltage of a night as I have a voltage gauge inside the bus and it usually holds around the 25.5v mark until I give the pillow a nudge.

The solar controller has built in 40a fuses on both the incoming +ve circuit and also the outgoing +ve circuit.and also supplies a set of spare fuses..

I fully understand about soldering heavy cable as I used to make welding cables and always soldered them so I am able to get the job done properly..

Sounds good. Just remember that the fuses at the battery are really there to protect the wiring, not the device, so fuses in the controller aren't going to protect the wiring between the battery and the controller.

oldbobsbus · Guru

The wire between the batteries and controller is only about 1m long the reg is mounted directly above the batteries..

This photo shows the set up when it was in 12v configuration I have since changes it over to 24v and put 2 new inverters in.

SnowT · Guru

On thing that is not shown is a High amperage Fuse-able Link..

It add's that little bit extra protection in case there is a short somewhere.. Generally about 100A +

Juergen

oldtrack123 · Guru

SnowT wrote:
On thing that is not shown is a High amperage Fuse-able Link..

It add's that little bit extra protection in case there is a short somewhere.. Generally about 100A +

Juergen

Hi

It seems adequate fusing has not been considered necessary

PeterQ

brian · Guru

Where would you suggest an adequate fuse be fitted? Bob doesn't appear to have any elv equipment other than the inverter connected to the batteries, and the wiring for that is only a couple of feet long and totally out of harm's way. Perhaps we should become really paranoid and fit a fuse between each link on the battery, maybe 2, just to be sure. Go take a look at your car, check out the wire from the battery down to the starter motor and see if you can find any fusing in that, and then think about which wire has the greatest propensity for damage. It is difficult to see if there is a fuse in the wire to the solar controller, but that is the only one that could possibly be needed here, and that is also such a short unencumbered run that fusing it also become a moot point. Remember, the fuse is there to protect the wiring, not the downstream equipment.

oldtrack123 · Guru

brian wrote:
Where would you suggest an adequate fuse be fitted? Bob doesn't appear to have any elv equipment other than the inverter connected to the batteries, and the wiring for that is only a couple of feet long and totally out of harm's way. Perhaps we should become really paranoid and fit a fuse between each link on the battery, maybe 2, just to be sure. Go take a look at your car, check out the wire from the battery down to the starter motor and see if you can find any fusing in that, and then think about which wire has the greatest propensity for damage. It is difficult to see if there is a fuse in the wire to the solar controller, but that is the only one that could possibly be needed here, and that is also such a short unencumbered run that fusing it also become a moot point. Remember, the fuse is there to protect the wiring, not the downstream equipment.

HI Brian

First off, I KNOW that a line fuse is to protect the wiring!

It can SOMETIMES also be used for equipment protection IF that is ALL that is on THAT circuit

In Bob's case, that could also be for ADDITIONAL inverter protection AND much easier to replace than the INBUILT inverter fuses

I consider a car /vehicle Starter feed as a totaly different situation [it is in an all metal situation] with little if any combustionable material near or likely to near those cables

I would have a main fuse on the DC positve cable as close as possible to the battery output.

The size to be determined by the cable CONTINIOUS rating OR the inverter CONTINIOUS rating ,which ever is the LEAST!

NOT IT'S rating as welding cable, which is usually around twice or more of it's continious rating

Some questions for Bob

Does your 240V installation meet ALL the requirements of AS/NZS 3000 AND AS/NZS 3001??

It is fixed wiring ,I hope the electrician doing it knew the requirements of the Standards!

PeterQ

-- Edited by oldtrack123 on Saturday 30th of November 2013 03:21:58 PM

-- Edited by oldtrack123 on Saturday 30th of November 2013 03:26:44 PM

SnowT · Guru

Hiya Brian..

Most car's Do not have a Link from the Battery to the starter, but they do have a Link on the main Battery to Wiring Loom connection..
My old ford XB had one.. the XF Panel van had one..

As a means of Protection..

If you have 4 battery's.. A Fusable Link Rated Higher than your Highest expected Current Load..
- A Link in the middle of the battery bank, is my suggestion..
-->> -- [-ve] B1>B2>>FuseLink>>B3>B4 [+ve] ++

ie.. if you have inverter that has a surge output of 2000w's and a battery voltage of 24 volt..
- A DC fridge of 70w..
- 40w of light's..

-- The Math's..
Invertor.. W = V x I
2000 = 24v x I
So the I = 2000/24
I = 83.3A's

Fridge and light's
[70 + 40] = 24v x I
110w/24v = I
I = 4.6A

If you use an convertor or such they are not 100% efficient so you have to allow about 10-15% higher load..

Total current Draw is :- [83.3 + 4.6] x 1.15 = I Total
Total Load is 101A

For a fuse Size.. Allow a 150A as a Max or 125A if you can get it..

Hope this help's

Juergen

Psst you should have a Fuse of some kind at your Inverter input..

-- Edited by SnowT on Saturday 30th of November 2013 03:41:36 PM

brian · Guru

If you have a close look at Bob's setup you will see that he has no elv house connections to the batteries and therefore no need to fuse any wires going to a separate distribution board, which I would agree that he should do if that were the case, and they were remote from the batteries That leaves just the 2 inverters to bother with, and they are very close to the batteries with buckley's chance of any damage to the wiring that could result in a short circuit. The inverters have their own internal overloads that will pop if anything goes wrong with them, so fusing is both a waste of time and another source of potential problems. My analogy with the starter cable shows that you don't need to fuse every wire just for the sake of it, and there's way more chance of damage to the starter cable than there is to Bob's inverter wires. Auto manufacturers have already done the deficit/benefit analysis on this subject and if they felt that fusing was needed, it would be there, especially in such a hot, hostile environment as under the bonnet. As I said earlier, if you are going to be concerned about protecting every bit of wire with an adequate fuse, then you will be putting one at each end of every interconnecting link on those batteries.