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I'm trying to understand the problems of using secondhand batteries

I'm trying to understand the problems of using secondhand batteries versus new batteries.

As an engineer I would always use the best battery you can buy, because it will save you money in the long term, I do understand the Financial restraints.

  1. Have you or would you, use batteries with second hand cells ?

  2. Have you experienced any problems ?

  3. What C Rating would you use ?

  4. What's your general opinion of using secondhand battery cells rather than new cells ?

  5. Would you give advise to anyone who uses second hand battery cells ?

The reason for this blog is we are experiencing a lot of calls where the DC bus voltage is too low where second hand cells have been used.



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Timothy Murray
Timothy Murray
Oct 26, 2021

A second Life battery has the same capacity as the equivalent 1st life battery, this is due to the 2nd life cell having been de rated in terms of its original capacity once removed from the vehicle.

So a 120ah battery is not specified as a 120ah battery, but is now specified as a 100ah battery .

Typically this accounts for the higher weight and size of a 2nd life battery versus 1st life battery


Most 2nd life vendors actually under specify the capacity as most 2nd life cells are between 5 to 10% higher in terms of actual capacity.


The use of 2nd life batteries opens up storage to people of lower income and is an extremely effective way…


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Marius Vincent
Marius Vincent
Oct 26, 2021

Timothy we design and manufacture lithium packs. We deal in first life but do supply limited 2nd life lifepo4.

Arnold depends on your BMS. Are you using 1st generation BMS type with a internal contactor? Does your BMS have a DC to DC converter?


We supply 3rd generation BMS lithium packs. We have developed our new 4th gen BMS recently.

Our 4th Gen BMS has AI processor and does intelligent smart interpack balancing and has internet access with monitoring down to cell level.

We also deploy new CANbus protocols remotely for new inverter integration etc over the air.


Getting back to balancing you will need a DC to DC converter 3rd gen BMS with dynamic charge voltages with CANbus or…


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Keith, ive tried to contact sunsynk and its representatives regarding my 2nd user experience and i think Sunsynk should be more understanding of those attempting DIY builds, who may be very skilled in that field I'm install a new sunsynk unit with a used BMW i3 battery of 30Kw. I've installed an excellent BMS (simpBMS) that manages and interfaces with the battery and keeps it healthy. This does not, however, interract with sunsynk, but realistically it does not need to as long as it keeps the battery healthy, safe and voltages balanced. The Sunsynk can then just manage the charge rates and voltage levels within limits imposed by myself Ive done the calculations on battery storage and the long term savings are not great enoug…


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Kevin Smith
Kevin Smith
Oct 25, 2021

Another question is what happens if you have LiPO4 batteries that are say 7 years old and due to degradation more power is needed. Can you add another new battery into the system?

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Blokeo
Blokeo
Oct 26, 2021
Replying to

Yes, indeed you can. For example if you have a 10 kWh system that presumably has derated to say 8 kWh, over a number of years. Adding a 5 kWh battery to that system would result in a total of 13 kWh being available. Just make sure that the operating voltage windows are the same: so acquire the batteries are from the same manufacturer, with the same chemistry and cell count. Both Battery modules will operate in perfect SoC sync with each other.

(This assumes a proper DC reticulation layout with correctly spec-ed cables and a large DC busbar.) This is a great way to utilise the long cycle life of Lithium batts.

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Marius Vincent
Marius Vincent
Oct 25, 2021

The BMS for 2nd life when it hits 0 percent the SOC on the BMS is also 0 percent. You would not know the difference in 1st or 2nd life as a user.

2nd life say 120ah is derated to 100ah but the BMS is programmed for 100ah to match.


However 2nd life cell balancing is a major problem as internal cell resistances changes faster the older the cell which means the BMS balancer takes much longer to balance. Worse is the cells don't discharge equally correctly unless you have an active balancer of high rating.


Cheers


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Blokeo
Blokeo
Oct 26, 2021
Replying to

Marius, I agree, the BMS is aligned with the current battery capacity 200Ah and determines the SoC from that point forward. So the 30% State of Charge remains true within the BMS across the cycle. At 30% SoC a 200Ah battery will still have 60Ah, and have a Bus Voltage of about 51V (if the battery module is a 16 cell construction – which holds up better under loads).

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