[RE-wrenches] Different LFP Batteries in Parallel

Mon Jul 14 09:39:55 PDT 2025

Steve,
Are you suggesting that even for like batteries in closed loop, that you
would add 20 minutes per paralleled battery of absorb time? This is much
longer than what I have understood various manufacturers to state. Thanks
for your input.
Howie Michaelson

On Mon, Jul 14, 2025 at 10:33 AM Steve Higgins via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Each battery has its own internal resistance. The higher the resistance,
> the longer it takes to charge. The issue is that chargers are
> voltage-based, and when charging multiple batteries in parallel, the
> lower-resistance batteries reach the target voltage sooner than the
> higher-resistance batteries.  These lower resistance batteries cause the
> voltage-based chargers to bypass the absorb stage, and this prevents the
> batteries that are a bit low and out of balance from achieving that voltage
> target.
>
> If your charger isn't a 2-stage charger, or you aren't holding the
> absorb timer long enough, or the charger is a "Smart Charger" and it can be
> made to think that the batteries are fully charged when they are not.
>
> The best way to avoid this:
>
> Wire parallel connections using positive/negative bus bars with
> equal-sized cabling. Do not parallel LFP batteries with only cables, and do
> not use the connections on the battery as a parallel connection.
>
> Use a charger that can be programmed for 2-stage charging.  Unfortunately,
> there aren't many of these out there...
>
> If you can't use a charger that operates in 2 Stage, you need to extend
> the absorb timers so the charger is supplying voltage to the BMSs of the
> parallel batteries, so all batteries have time to get to full balance.   We
> typically suggest 20-30 minutes of absorption time per parallel-connected
> battery up to about 3 to 4 hours.   For that bank with 10 batteries, I
> would set the absorb timers to at least four, if not five, hours.
>
> You should also disable any end amp/return amp settings, as these are
> often not programmed with a time delay and will result in the charger
> bypassing the absorption timer.
>
> In general, the more the customer works with the bank, the more often
> they'll have to balance.
>
> I would start by checking SOCs and Voltages every 90 days. If you notice a
> difference of more than 0.3 volts or a spread of more than 4-5% points, I
> would separate each parallel battery and boost each battery at Bulk/Abs
> Voltage for at least one hour of abs time.   Suppose the bank is way out of
> balance. In that case, it may take a few cycles(6-10) to achieve that
> balance, even so if it's been out of balance for a long time, you may never
> again achieve full balacne, in those cases I would manually balance more
> often every 1-2 months instead of 6-12 months.
>
>
>
> *Steve Higgins* *⋅* *Technical Services Manager*
> *t* +1.902.597.4020   *m* +1.206.790.5840
> *f* +1.902.597.8447   *e* steve at surrette.com
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>
> On Mon, Jul 14, 2025 at 6:54 AM jay via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
>> Hi Steve,
>>
>> Maybe you can elaborate as to how this could happen.
>>
>> Let’s say all the batteries are designed for a 57.4 usual charge voltage
>> with an acceptance range of around 56-58.4v.
>>
>> If you’re charging 10 batteries to 57.4v, the most that any one battery
>> gets to is 57.4.  If one gets full before others then it stops accepting
>> charge, but at no point in the system does any battery see more than 57.4.
>>
>> Iv’e seen this with same 10 batteries all from one OEM all in one system,
>> all new.  They get off just a little and some take longer or shorter to
>> charge fully to 100% but the voltage never rises above what the inverter or
>> CC is set to. I don’t understand how this would cause damage or issue.
>>
>>
>>
>> thanks
>>
>> jay
>>
>>
>>
>> On Jul 14, 2025, at 7:46 AM, Steve Higgins via RE-wrenches <
>> re-wrenches at lists.re-wrenches.org> wrote:
>>
>> I wouldn't do it... By doing so, you'll likely void any manufacturer's
>> warranty.
>>
>> If you must do it, I would manually balance the old/new banks every 60-90
>> days. This involves separating the parallel strings of batteries and
>> charging each string independently to achieve a full charge. If you don't
>> do this, the different ages can cause imbalances, which will lead to
>> problems with the BMS, regardless of whether it is in open-loop or
>> closed-loop configuration.
>>
>>
>>
>> *Steve Higgins* *⋅* *Technical Services Manager*
>> *t* +1.902.597.4020   *m* +1.206.790.5840
>> *f* +1.902.597.8447   *e* steve at surrette.com
>> ------------------------------
>> CONFIDENTIALITY: The information transmitted herein is intended only for
>> the addressee and may contain confidential, proprietary and/or privileged
>> material. Any unauthorized review, distribution or other use of or the
>> taking of any action in reliance upon this information is prohibited. If
>> you receive this email in error, please contact the sender and delete or
>> destroy this message and all copies.
>> ------------------------------
>>
>>
>>
>> On Mon, Jul 14, 2025 at 4:09 AM Jason Szumlanski via RE-wrenches <
>> re-wrenches at lists.re-wrenches.org> wrote:
>>
>>> There are never-ending videos on YouTube showing different brands and
>>> capacities of LFP batteries working in parallel. I have never tried it
>>> myself, and every instinct says don't do it, but I have a situation where
>>> it would be very helpful.
>>>
>>> I have a client with an older 60 kWh battery that is discontinued and
>>> incompatible with any other battery from the same manufacturer. It has a
>>> BMS with a 51.2 volt nominal rating. They need another 20 kWh of capacity.
>>> It is too costly to scrap the existing battery and start over, and the
>>> existing battery has been tested and confirmed to hold very close to its
>>> rated capacity.
>>>
>>> What is the real deal on mixing and matching? Is anyone doing this
>>> safely and successfully? I wonder what the battery manufacturers say about
>>> it.
>>>
>>> Jason Szumlanski
>>> Florida Solar Design Group
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