Even discharge and recharge of batteries

[Mr Mousy]

I would like some advice please.

I originally had one KS Energy 200ah lithium battery connected to my Hymer S840 using 35mm cables. The battery has completed about 40 cycles. I then bought a brand new identical battery and had it professionally installed by a well known company using 35mm cables (positive and negative cables the same length) both pairs of cables are fused.
All electrical connections are sound and everything works.
The attached diagram shows the existing connections - the original battery discharges / recharges - using 70% ish of the current the second battery using about 30%.

I would like both batteries to discharge / recharge at about the same rate.

What do I need to do?

Thank you for reading to the end of this post!

 

[Jimbohorlicks]

I think you just need to connect them to a couple of bus bars or connect the reich box positive + ve to the new battery positive( not to the old battery positive) so its drawing from both batteries rather than the older one preferentially.

 

[stevewagner]

I was just writing virtually the same but would add if you use bus bars make sure all the cables are the same length.

 

[Jev88]

Generally, battery banks should be same type, similar size and same age.

 

[headlight]

I would like some advice please.

I originally had one KS Energy 200ah lithium battery connected to my Hymer S840 using 35mm cables. The battery has completed about 40 cycles. I then bought a brand new identical battery and had it professionally installed by a well known company using 35mm cables (positive and negative cables the same length) both pairs of cables are fused.
All electrical connections are sound and everything works.
The attached diagram shows the existing connections - the original battery discharges / recharges - using 70% ish of the current the second battery using about 30%.

I would like both batteries to discharge / recharge at about the same rate.

What do I need to do?

Thank you for reading to the end of this post!View attachment 992863

You want the negative of each battery to a bus bar, the positive of each battery to a bus bar, all 4 cables identical length, rating and connectors.
Then go from the bus bars to your REICH.
That way the batteries will share charge and discharge equally (pretty much).
The way you've wired it battery 1 will always do most of the work.
Likewise if you changes it so battery 2 went to the REICH box then battery 2 would do most of the work.
Oh and obviously suitably cabled and fused.

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[Lenny HB]

Agree with the above you should never wire Lithiums the way you have.
Also don't believe everthing the BMS tells you, to get an accurate reading from the BMS's you need to charge the batteries to 100% then discharge them to 50% 3 or 4 times to get the BMS's reading accurately.

 

[pappajohn]

You've wired it as you would a pair of lead acids.
Rewire as Jimbo says.

 

[Mr Mousy]

Thank you for your advice. For the record I didn’t install them - I paid a reputable company to do the work

 

[Lenny HB]

Thank you for your advice. For the record I didn’t install them - I paid a reputable company to do the work

Oh dear!

 

[Mr Mousy]

Oh dear!

Exactly

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[stevewagner]

I’d ask for a refund

 

[AdrianChen]

You've wired it as you would a pair of lead acids.
Rewire as Jimbo says.

Not sure I would wire Lead Acid or any type like that.....

 

[LostInSpace]

I would like both batteries to discharge / recharge at about the same rate.

What do I need to do?

Stop worrying about it.

Unless you are always using a very high power inverter with high power loads, The batteries will be fine

Even then you may never notice the lose of lifespan of the batteries.
If one does weaken the other will pick up the load.

 

[autorouter]

The basic principle is that you need exactly equal voltage drop along the wires to each battery. There are a couple of ways to do this. One way, as others have said, is to use two busbars and equal length wires.

Another way is to wire the Reich box positive to one battery, and the Reich box negative to the other battery. Then link the two battery positives together, and link the two battery negatives together using a link exactly the same length and thickness as the positive link. That will ensure that the voltage drops to the two batteries are equalised.

The wires between the batteries and the Reich box do not need to be any particular length and thickness. It's the two link wires that need to be equal.

I think your setup is almost right, just move the negative Reich box wire to the other battery - assuming your link wires are equal.

 

[Carpmart]

Generally, battery banks should be same type, similar size and same age.

This is lithium.. same IR of the batteries is useful of course, but different size batteries, if wired correctly, doesn’t matter!

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[jumar]

My batteries discharge at different rates...mainly due to the batteries being in different locations... therefore the cables are the same in diameter but differ in length...I was aware that this could happen, but was left with no real option... Initially concerned but not so now...could make cables in battery one the same as battery two but that would just be a dead loop, which I believe is a pointless exercise...

 

[Carpmart]

I would like some advice please.

I originally had one KS Energy 200ah lithium battery connected to my Hymer S840 using 35mm cables. The battery has completed about 40 cycles. I then bought a brand new identical battery and had it professionally installed by a well known company using 35mm cables (positive and negative cables the same length) both pairs of cables are fused.
All electrical connections are sound and everything works.
The attached diagram shows the existing connections - the original battery discharges / recharges - using 70% ish of the current the second battery using about 30%.

I would like both batteries to discharge / recharge at about the same rate.

What do I need to do?

Thank you for reading to the end of this post!View attachment 992863

Belt and braces would be equal length/diameter wires from each battery back to reich box (don’t know what this is) then link wires across the cells positivepositive and negative:negative terminals; again equal length/diameters for the links… it’s over the top as other have said, bus bars would give you a simpler way of doing it.

Why did OffGrid wire this as per your drawing Martin? Seems a very strange/poor approach to me?

 

[AdrianChen]

My batteries discharge at different rates...mainly due to the batteries being in different locations... therefore the cables are the same in diameter but differ in length...I was aware that this could happen, but was left with no real option... Initially concerned but not so now...could make cables in battery one the same as battery two but that would just be a dead loop, which I believe is a pointless exercise...

If they are Lithiums I doubt it will make a huge difference to their lifespan if they are fully charged regularly, and not frequently run at max output currents for long periods.

I think with LA there are potentially more risks of shortening their combined lifespan, which perhaps is more noticeable with "leisure" batteries that only manage around 300 cycles, vs Lifepo4.

It's good to have any bank wired properly though, and it will certainly help to get the best out of them.

 

[jumar]

If they are Lithiums I doubt it will make a huge difference to their lifespan if they are fully charged regularly, and not frequently run at max output currents for long periods.

I think with LA there are potentially more risks of shortening their combined lifespan, which perhaps is more noticeable with "leisure" batteries that only manage around 300 cycles, vs Lifepo4.

It's good to have any bank wired properly though, and it will certainly help to get the best out of them.

They are Lithium, and due to the initial cost, I tend to treasure them...they are our lifeline for off grid usage...both quality Roamer brand.

 

[Wanders from Glos]

Thank you for your advice. For the record I didn’t install them - I paid a reputable company to do the work

How annoying, I hope they don’t specialise in off grid work. It would be quite worrying.

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[StillGoing]

I would like some advice please.

Take it back to the well known company who professionally installed it and ask them to wire it properly. Sadly, you are far from alone in relying on so-called professionals and ending up with a poor result. Even if they don’t install bus bars, wiring the load to the positive of one battery and the negative of the other battery is fairly basic.

Happy Christmas!

 

[eddie]

Thank you for your advice. For the record I didn’t install them - I paid a reputable company to do the work

Who?

 

[headlight]

Thank you for your advice. For the record I didn’t install them - I paid a reputable company to do the work

Not that reputable then unfortunately and probably charged you handsomely for a poor install.

 

[autorouter]

My batteries discharge at different rates...mainly due to the batteries being in different locations... therefore the cables are the same in diameter but differ in length...I was aware that this could happen, but was left with no real option... Initially concerned but not so now...could make cables in battery one the same as battery two but that would just be a dead loop, which I believe is a pointless exercise...

The principle is to have equal voltage drops to each battery. Usually that means equal wire lengths, so that the wire resistances are equal. But it's the equal wire resistances that matter. A wire of half the length and half the cross-sectional area would be the same resistance.

So you could replace the long thick wire with a shorter thinner wire so it has the same resistance as the long thick wire. I know it's counter-intuitive to introduce extra resistance into a battery charging setup. but you would be balancing the charging amps by adjusting the resistances of the wires so that they are equal.

It's possible to do more elaborate calculations to equalise the resistances, if you wanted to use wires of other cross-sectional areas, but 'half the CSA means twice the resistance' is a good start.

 

[bigtwin]

I know it's counter-intuitive to introduce extra resistance into a battery charging setup. but you would be balancing the charging amps by adjusting the resistances of the wires so that they are equal.

Indeed it is and it’s why many ‘traditionalists’ struggle to accept/embrace the hybrid battery arrangement that narrowboaters use where they use the resistance in the alternator wire to limit the current being feed to the LiFePO4 batteries.

When I first got my NB I had a charging problem with my lead acid bank. It turned out that there was a high resistance on the earth path that was reducing the voltage seen at the battery busbar. Once rectified, charging was all good.

Subsequently I replaced the LA bank with a hybrid LiFePO4/LA bank. On commissioning it was clear that there was insufficient resistance in the charging path such that the alternator was being worked too hard (this wasn’t much of a surprise as, curiously, the alternator lead was a massive 95mm2). Once the alternator cable had been replaced by a longer and smaller CSA (adding resistance to the charge path), the charging current had been reduced to acceptable alternator running temperature.

Thus, in the one setup (LA) resistance was the enemy of effective charging and in the other setup (LA/LiFePO4 hybrid), resistance was the friend of effective charging.

Ian

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[CAB96]

Is it worth the hassle?

The lifespan of these batteries is thousands of cycles. Do you think you will still be using them when they get to End Of Life?

 

[bigtwin]

Is it worth the hassle?

The lifespan of these batteries is thousands of cycles. Do you think you will still be using them when they get to End Of Life?

The hassle of moving one lead from one battery post to another? Not much hassle/effort in the grand scheme of things.

Ian

 

[AdrianChen]

Indeed it is and it’s why many ‘traditionalists’ struggle to accept/embrace the hybrid battery arrangement that narrowboaters use where they use the resistance in the alternator wire to limit the current being feed to the LiFePO4 batteries.

When I first got my NB I had a charging problem with my lead acid bank. It turned out that there was a high resistance on the earth path that was reducing the voltage seen at the battery busbar. Once rectified, charging was all good.

Subsequently I replaced the LA bank with a hybrid LiFePO4/LA bank. On commissioning it was clear that there was insufficient resistance in the charging path such that the alternator was being worked too hard (this wasn’t much of a surprise as, curiously, the alternator lead was a massive 95mm2). Once the alternator cable had been replaced by a longer and smaller CSA (adding resistance to the charge path), the charging current had been reduced to acceptable alternator running temperature.

Thus, in the one setup (LA) resistance was the enemy of effective charging and in the other setup (LA/LiFePO4 hybrid), resistance was the friend of effective charging.

Ian

Don't think it's just NB's and Hybrids - I've seen a few boats where the cables from a large domestic bank were limiting the max current demand on the standard puny alternators on a lot of marine engines. As long as they're not too small, or the bank too large it works!

 

[AdrianChen]

Who?

Hopefully a competitor!

 

[Raul]

So you could replace the long thick wire with a shorter thinner wire so it has the same resistance as the long thick wire.

Actually, this is the preferred method in trying to equalise the resistance path in both routes.
I also do have batteries located in 2 different places, location 1 with 2 batteries paralleled and then to buss with 35mm2 1.2m and location 2 3m with 50mm2. All contribute almost equal.
By voltage drop you will not be able to equalise the flow, but, by resistance conductor you can, as the source is equal potential and end result will be impeded by equal resistance- equal flow, or different resistance-different flow. Both pats have equal potential at source.

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