That is right. Then I will know that because my BMS boards will give a
warning. But if that point never comes I will not hear from my BMS and
know I could have been fine without it. If I stay above 30% SOC and
below 100% SOC small cell variations won't matter.
In any case I'm not convinced yet that a pack of non-abused cells
needs to be balanced every charge. I believe any balancing can be done
every year or so. Understand my data is for cells in a single battery
box so I don't have data on cells at grossly different temperatures.
warning. But if that point never comes I will not hear from my BMS and
know I could have been fine without it. If I stay above 30% SOC and
below 100% SOC small cell variations won't matter.
In any case I'm not convinced yet that a pack of non-abused cells
needs to be balanced every charge. I believe any balancing can be done
every year or so. Understand my data is for cells in a single battery
box so I don't have data on cells at grossly different temperatures.
Cor van de Water <[email protected]> wrote:
> David,
>
> If you never balance your cells then
> I am afraid that you will find out the answer
> to your assumption on the occasion that you
> have driven too far for the lower cells,
> then you will know how much unbalance there was.
> The curve for LiFePO4 is so flat that you have
> no clue if a cell is sitting at 90% or 60% or 30%
> until it suddenly drops like a brick.
> You can get all cells very well balanced by
> either top balancing (charging until a fixed
> voltage that is at least slightly above the knee
> for example 3.6V (note: you must measure *each*
> cell, do not rely on charging the whole string
> to N * 3.6V as then one cell may be at 4.5V while
> most are still hovering around 3.4V!)
> or by bottom balancing, discharging each to a
> fixed voltage - for example 2.5V
>
> Regards,
>
> Cor van de Water
> Director HW & Systems Architecture Group
> Proxim Wireless Corporation http://www.proxim.com
> Email: [email protected] Private: http://www.cvandewater.com
> Skype: cor_van_de_water IM: [email protected]
> Tel: +1 408 383 7626 VoIP: +31 20 3987567 FWD# 25925
> Tel: +91 (040)23117400 x203 XoIP: +31877841130
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Beh=
alf Of David Nelson
> Sent: Thursday, February 17, 2011 8:49 AM
> To: Electric Vehicle Discussion List
> Subject: Re: [EVDL] Li-Ion balancing, was: conversion reliability
>
> I think a couple of other questions that need to be asked is what voltage=
were they being charged to and how far were they discharged?
> We'll see over time but so far I'm not seeing a need to balance my pack b=
ased on the data I've collected on it for the past 8 months. The difference=
might be that I'm only charging to 3.485Vpc and not to the stratosphere of=
voltages.
>
> Maybe the reality with LiFePO4 cells is more one of if you charge them to=
more than 3.6vpc they must be balanced at end of charge and if you stay be=
low that, say 3.5vpc, then they don't have to be balanced or that they only=
need to be balanced once every 1-5 years as a standard maintenance procedu=
re.
>
> On Tue, Feb 15, 2011 at 11:50 PM, Jack Murray <[email protected]=
> wrote:
>> The question is how balanced were the cells to begin with, when the
>> pack was constructed?
>>
>> --- On Tue, 2/15/11, Cor van de Water <[email protected]> wrote:
>>
>>> From: Cor van de Water <[email protected]>
>>> Subject: [EVDL] Li-Ion balancing, was: conversion reliability
>>> To: "Electric Vehicle Discussion List" <[email protected]>
>>> Date: Tuesday, February 15, 2011, 11:16 PM Mike, This is very
>>> important experience data, thanks for posting this!
>>> Contrary to other claims (granted, for other Li-ion
>>> chemistry)
>>> there is a need for balancing, if your experience shows that one cell
>>> may take 20Ah to balance against another cell in your pack...
>>>
>>> 55 miles at 250Wh/mi is 13750Wh.
>>> On your 162V nom pack that means at least 85Ah or at least 85% DoD.
>>> That is indeed uncomfortably low,
>>> especially if you are not sure that
>>> your cells are balanced.
>>>
>>> Cor van de Water
>>> Director HW & Systems Architecture Group Proxim Wireless Corporation
>>> http://www.proxim.com
>>> Email: [email protected]
>>> Private: http://www.cvandewater.com
>>> Skype: cor_van_de_water IM: [email protected]
>>> Tel: +1 408 383 7626 VoIP: +31 20 3987567 FWD# 25925
>>> Tel: +91 (040)23117400 x203 XoIP: +31877841130
>>>
>>> -----Original Message-----
>>> From: [email protected]
>>> [mailto:[email protected]]
>>> On
>>> Behalf Of Mike Nickerson
>>> Sent: Wednesday, February 16, 2011 11:04 AM
>>> To: 'Electric Vehicle Discussion List'
>>> Subject: Re: [EVDL] conversion reliability
>>>
>>> I can charge at work, so I'm only using about 50% each way. I
>>> wouldn't be comfortable with what it would do to the batteries round
>>> trip.
>>>
>>> I drove 55 miles on the pack once. That was about 3-4 miles too
>>> many.
>>> I couldn't hear the alarm buzzer from the miniBMS at highway speed so
>>> I noticed the batteries were in distress when I got to my driveway.
>>> By then 2 cells were at 0 volts. I charged them up right away,
>>> babied them during a slow charge and kept close track of the
>>> temperature of the cells. They seemed to recover OK, but I'm still
>>> watching them.
>>>
>>> I also learned that my cells weren't as balanced as I previously
>>> thought. I had a few other cells that were about 20% lower than the
>>> top cells. That's when I went out and bought the top-up charger in
>>> my last post. That charger does a really nice job of pointing out
>>> how low your cells are. Kind of makes you gulp when it puts in 20Ah
>>> just after the main charging cycle finishes. On a full cell, it
>>> measures less than 1Ah it tried to put in.
>>>
>>> I also have found recently that my front disk brakes are dragg
-- =
David D. Nelson
http://evalbum.com/1328
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