question

Klaus avatar image
Klaus asked

Lithium - mixing old and new batteries

Is it OK to expand a one year old set of two 330Ah Victron Lithiums Smarts with one new 330Ah Lithium Smart to upgrade to 990Ah capacity?


The older set has done somewhere between 50-100 cycles.

The installation is on a sailboat with discharging and charging currents typically less than 0.1C


If yes, after how many cycles/years would you no longer do an expansion in such a way?


Would the answer be any different if I do redundant load and discharge buses, i.e. every Lithium battery gets its own BMS + ATC Battery Protect + ATD Battery Protect?


Thank you!

Klaus.

Lithium Battery
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2 Answers
Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) answered ·

Hi @Klaus Matzka,

I don't have a definitive answer. It depends for example on how deep were your discharges and typically also on internal cell resistance.

50-100 cycles down to 0% is different compared to discharge down to let's say 50%. In case of the latter the older battery is almost like new and I would give it a try by charging all batteries up 14.20V and let them balance. Then measure cell resistance (the 330Ah can be opened though not advisable!)

I'm guessing here but think that you have a GX device and either a VE.Bus BMS or a Lynx Smart BMS. I think it will not work with multiple BMS. Also, if you do have a GX device, it will only control the 'main' battery.


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Klaus avatar image Klaus commented ·

Hi @Stefanie


Thank you for your fast and helpful response! :-)


I believe you are right: if the batteries are discharged lightly for a few dozen cycles it should be ok to add another battery of the same brand and model.


BTW, how does one measure the internal resistance? Just by measuring the resistance between the positive and negative poles?


Some more background: to keep the original question simple I have been pretending the system is already a year old. But I am actually currently planning the system now, and was looking back from the future… ;-)


The design:


I am planning to built a redundant load/charge bus, with one smallBMS per Lithium Smart Battery, plus one Smart Battery Protect for the charging side, and one for the loads side. No GX devices, no Lynx Smart BMS. I do not see the benefits of the more complex, more expensive and stand-by current sucking system. No need for remote monitoring. If I leave the boat for longer, I shut everything down completely, including BMSes, battery monitors, etc.


Why am I planning to install a dual bus system: to reduce the risk of one Lithium failing momentarily for whatever reason and taking down the whole DC system in a sudden leaving the boat without any energy. Imagine the auto-pilot (silently) failing while sailing single handedly during the night…


Back to the topic:


after some more research I came to the conclusion that adding a battery later might mean that I have to cope with batteries with different internal resistances.


Is this really a serious issue?


My guess: it would mean the new battery (of same brand/model/chemistry) is doing more work during load/discharge. Thus it would age faster than the older batteries. I conclude that would mean old and new batteries are coming closer in „age“ over time and then age further together.


Do you think this is a correct assessment?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ Klaus commented ·

BTW, how does one measure the internal resistance? Just by measuring the resistance between the positive and negative poles?

Measuring between the positive and negative battery poles is not accurate enough to determine internal cell resistance. It is much more accurate if you measure directly over the individual cells. There are plenty of videos on youtube on how to do that and understand how the internal resistance change depending on several factors.

after some more research I came to the conclusion that adding a battery later might mean that I have to cope with batteries with different internal resistances.


Is this really a serious issue?

It can become a serious issue. Cells get unevenly charged/discharged, which leads to cell imbalance and probably a shorter lifespan.

To exaggerate this:

Imagine a cell with an internal resistance of 0.15mΩ and the other cell with an internal resistance of 0.30mΩ. Both values are initially very small. However, the second value is double that of the first cell. Now Ohm's Law comes into play...

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Klaus avatar image Klaus Stefanie (Victron Energy Staff) ♦♦ commented ·

I try to relate that to my original question:

You are describing the resistance differences between cells _within_ a Lithium Smart battery. I understand that this is an issue.

But how is this imbalance within a battery effected by adding another Lithium Smart battery (with its own cell balance/imbalance within) to a parallel installation of Lithium Smart batteries. Cell balancing within a battery should be unaffected by what is going on outside of itself, isn’t it? Or do I miss something here?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ Klaus commented ·

The imbalance will most likely not be affected by adding another lithium. But the overall performance might be affected. The entire battery is only as good as the weakest cell in it (edit: the last sentence is true for a single battery - cells are in series to build a 12.8V battery).

To wrap this up:

Batteries with different capacities can be connected in parallel without any problems. The different capacities then add up. Of course, the ideal situation would be if all the batteries were in the same condition.

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gn-ro avatar image
gn-ro answered ·

If the internal resistance of all batteries is similar, then you can use them in parallel with no problems. You can even use batteries with different capacities as long as they are of same technology (Li-Ion with Li-Ion or LifePO4 with LifePO4, etc.).

Remember, even a slight difference in voltage will overcharge the lower voltage one and discharge the higher voltage battery but this is not a problem if the batteries are using the same max and min voltage EXACTLY. The initial balance is not a problem, but try to add the newer battery close to the voltage of the "old" already running ones.

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Klaus avatar image Klaus commented ·
Hi @gn_ro , thank you for your explanation! Very much appreciated!
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kurt-s avatar image kurt-s commented ·

When adding a second (new) Victron Smart LifePO4 battery in parallel with an older one, how does one be sure the batteries are using the same max and min voltage?

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