Mixing Amps on a House Bank?

[roland stockham]

Originally Posted by foggysail

Can you provide examples? Now as to connecting two batteries in parallel where there is only a 2v difference, hard to see where huge currents will occur when one considers each batteries internal resistance. The term impedance is misused when applied to DC, it only applies to AC. Sure, without considering all of the circuit resistances, one can calculate huge currents screaming toward infinity even with only 1 volt to work with. But of course we here are not in an electrical engineering circuit theory class room.

Now a comment on the so called warning not to hot switch. I presume that warning is to prevent arcing when a DC current is interrupted especially if there is any significant inductance which the current passes through. Not only will an arc occur but there can be enormously high voltages developed that are highly destructive.

I did not think I used the term impedance but possibly I did, sorry and you are right I should have said resistance. Anyway high current issue. A 400a/hr FLA stores 4800 watt hours IxV=W. so at 50% DDC it has the potential to take up half that eg 2400w.


The terminal voltage will have dropped more than 1v but let us assume it is at 12v.



The fully charged battery next to it is at 13v.


If you connect the two together the charged cell will attempt to balance the discharged one as fast as possible.



Limits are the resistance of the connection and the ability of the battery to provide current controlled by its internal resistance.



Clearly as soon as this happens the plate voltage on the discharged battery will rise so we are not going to see all that energy rush from one to the other. However at the moment of connection you do have 1v between them and a switch with a continuous rating of 3-500a.



Take the lower figure of 300a. On a 12v circuit in order not to overheat how much energy can that switch dissipate? It is plastic with no heat sink remember. If it's resistance is 1 ohm it would generate 3.6kw of heat, 0.1 ohm 360w, 0.01 36w. 0.001, 3.6. OK that may get a little to hot but not immediately fry so lets say the circuit resistance is in that order. Closing the switch we have an instantaneousness current in the order of at least 1x0.001 or 1000a. Result arcing and pitting of the contacts. That will very quickly increase the resistance in the switch. as it rises it generates more and more heat, eventually a thermal overload overload is inevitable.


So using these for the purpose for which they are designed ie fire/isolation switches is not only safe but absolutely recommended. But using them in a situation where they switch large currents on and off on a regular basis is a potential fire hazard and best avoided. And, yes, I have seen the evidence on the photographs we where show during a coastguard marine fire rescue course but I don't have it and don't know where you would find it.


Like everything else on hear it is just one persons opinion. I have given the reason why I believe it to be the case. By all means either check other sources or ignore what I say.

[ramblinrod]

Originally Posted by foggysail

The term impedance is misused when applied to DC, it only applies to AC.

Not technically correct.

Using the term “impedance” for a DC circuit is valid, despite the impedance being almost completely resistive.

[hzcruiser]

Originally Posted by roland stockham

And, yes, I have seen the evidence on the photographs we where show during a coastguard marine fire rescue course but I don't have it and don't know where you would find it.

There is no doubt you have seen photos of burnt up boats and no doubt the source might have been identified as the 1-2-both switch. We're all good here.

A capital-case "however", though: I highly doubt that the switch overheated because of the short pulse you'll get when turning it on or off under normal circumstances.

Unusual circumstances would be to try and cut off a run-away current (say a panic reaction because the wires had no fuse and are starting to emit smoke due to a short), then even more panic when you hear the sizzle and see flashes and arcing that might be visible through the plastinc housing of the switch, letting go of the switch while the arcing continues (rather than being decisive and fully turn the knob in a swift motion). Only then could I imagine the switch catching fire.


Connecting two batteries of different capacacity or SOC is like turning a large consumer on or off via the switch: it will over time lead to pitting and discoloration at the leading edge of the contact surface but the rest of the contacts will be fine and not affect the resistance. Below is a pic of the wiper after years of regular use.





Coming back to the OP's question, I would say, yes, of course you can mix batts of different sizes together, if your exiting batts are still in good nick (measure at least the voltage). And yes, the usage profile will be different but overall it will increase your capacity and that's what you want to achieve. Simple.

[GrowleyMonster]

Originally Posted by ramblinrod

Not technically correct.

Using the term “impedance” for a DC circuit is valid, despite the impedance being almost completely resistive.

Rod is right. Resistance is one type of impedence, just like capacitance and inductance.

[Tikka]

there is no problem to mix different capacity in the house bank ,
led acid batteries are voltage control device,
when charging - voltage is limited, when discharging voltage is directly proportional
to capacity available in the bank.
each battery only takes as much current as need it, regardles how much the others need.

[noelex 77]

Batteries of different capacity are not a great problem. The larger issue is that they will be batteries of different ages. It would be worth doing a capacity test on the old batteries to try and gauge what condition they are in.

It sounds like you will be you are installing a new battery box and wiring etc. If doing this, I would consider installing a seperate battery switch so the two banks can be isolated.

This is often criticized, I think by people who do not understand the concept and imagine that means you are always going to run the batteries as a split bank. If used intelligently it has a lot of advantages. To mention just one advantage, you will be able to monitor the banks separately.

The biggest danger of mixing old and new batteries together is when the old batteries die by the time this is obvious they have damaged the new bank as well. The ability to monitoring the two banks separately helps identify the early signs of the old batteries deteriorating.

[hzcruiser]

Originally Posted by noelex 77

The biggest danger of mixing old and new batteries together is when the old batteries die by the time this is obvious they have damaged the new bank as well. The ability to monitoring the two banks separately helps identify the early signs of the old batteries deteriorating.

Correct. The best way is to monitor each battery individually including Ah in and out and log it all into a database. Then you can look at some spiffy graphs once in a while and check if one batt is dropping out of line.

[john61ct]

Originally Posted by roland stockham

The fire hazard can be an issue when you can connect 2 batters with different states of charge. The inrush current even though only quite brief causes damage to the contacts increasing the resistance. Over time this can lead to the switch overheating and melting

Widely believed, but poppycock IRL.

Assuming you use quality switches like Blue Sea (BEP, Marinco), at appropriate ampacity.

And of course use them properly.

> can also blow alternator diodes

I consider using manual switches to direct charge currents to be poor design.

But Blue Sea's AFD feature is designed to prevent that problem.

[foggysail]

Originally Posted by GrowleyMonster

Rod is right. Resistance is one type of impedence, just like capacitance and inductance.

That hurts.... Yes, it can be used to describe just the resistive element with the reactives equal to zero with the result having a zero phase shift. In engineering practice I have never actually encountered the term used in DC circuits.

[LongRange]

Originally Posted by foggysail

That hurts.... Yes, it can be used to describe just the resistive element with the reactives equal to zero with the result having a zero phase shift. In engineering practice I have never actually encountered the term used in DC circuits.

It's a bit like saying the DC circuit is really AC, with a frequency of 0Hz

[Cadence]

I guess if someone does not want to use impedance for a DC circuit, they need to break things down to capacitive reactance, inductive reactance and resistance. The first two are definitely not resistance since they vary with time. The original conversation was battery related so I guess maybe once the batteries have topped out, it at that point might be called resistance. I know splitting hairs.

[scottgledhill]

I have different sized lead acid house batteries. I noticed that charging seemed to cease once the smaller battery was fully charged. I have now separated them with a switch and have had no issues.

[Waveform]

I’m an electrical engineer. It makes no difference. You can combine any size batteries. Just have to be the same chemistry. It’s that simple.

[Paul Emmanuel]

I recently had to replace my entire battery bank of 4 and was told, in no uncertain terms by my mechanic, to NOT mix types.

[john61ct]

Everyone on the planet myself included overestimates their knowledge in most topic areas. No insult intended, sorry if you took it that way.

EE is a very wide field.

DC / alternative energy, and especially marine electrics is a very specialized niche.

Few EEs are experienced in that niche.

Your statement completely ignored a critical factor that most consider very important.

I advise you delete it or at least revise it so as not to mislead those who are starting out.