Forums

Charging "tall" battery stacks

Started by Don Y December 21, 2016
I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here.

"In theory", as all of the batteries see the same load, they should all
degrade in lock step.

"In practice", you expect some variations.

How much can I potentially gain (life, capacity, etc.) if I design a
"stacked charger" (e.g., 4 floating 12V chargers) instead of a
single 48V charger?

At the very least, it should facilitate replacing single batteries
(instead of replacing the entire stack at the same time) -- ?
On 2016-12-21, Don Y <blockedofcourse@foo.invalid> wrote:
> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here. > > "In theory", as all of the batteries see the same load, they should all > degrade in lock step. > > "In practice", you expect some variations. > > How much can I potentially gain (life, capacity, etc.) if I design a > "stacked charger" (e.g., 4 floating 12V chargers) instead of a > single 48V charger?
not a whole lot. if you really want to balance them you should be using 2V cells. you'd need 24, and 24 chargers. -- This email has not been checked by half-arsed antivirus software
On 12/20/2016 11:41 PM, Don Y wrote:
> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, > here. > > "In theory", as all of the batteries see the same load, they should all > degrade in lock step. > > "In practice", you expect some variations. > > How much can I potentially gain (life, capacity, etc.) if I design a > "stacked charger" (e.g., 4 floating 12V chargers) instead of a > single 48V charger? > > At the very least, it should facilitate replacing single batteries > (instead of replacing the entire stack at the same time) -- ?
If you want imbalance, replace single batteries. If you want balance, simplest is to build a voltage divider and hook it to the batteries. Set them at 1% of the load and don't worry about it. I pulled the 1% out of my ass. Depends on charge current load current, duty factor...pick a number. This assumes they're charged frequently. If not, put the load on a connector that you can plug in when you charge 'em. For batteries that are disconnected when charged, charge them in parallel, with some current limiting light bulbs between 'em. I did that with my bicycle light. 4 lithiums all brought out to a connector. The bike runs them in series. Charged in parallel, again with some current limits so they don't blow up if mismatched.
On Wed, 21 Dec 2016 00:41:34 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here. > >"In theory", as all of the batteries see the same load, they should all >degrade in lock step. > >"In practice", you expect some variations. > >How much can I potentially gain (life, capacity, etc.) if I design a >"stacked charger" (e.g., 4 floating 12V chargers) instead of a >single 48V charger? > >At the very least, it should facilitate replacing single batteries >(instead of replacing the entire stack at the same time) -- ?
Eons ago I designed a chip to charge-control multi-cell LiIon batteries. It used shunting MOS FET's across each cell and a MUX'd sensing system to "balance" the pack. Customer was California Micro Devices (Tempe, near MicroRel, also one of my customers ;-). I'm not sure if they even still exist. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
On 12/21/2016 3:45 AM, Jasen Betts wrote:
> On 2016-12-21, Don Y <blockedofcourse@foo.invalid> wrote: >> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here. >> >> "In theory", as all of the batteries see the same load, they should all >> degrade in lock step. >> >> "In practice", you expect some variations. >> >> How much can I potentially gain (life, capacity, etc.) if I design a >> "stacked charger" (e.g., 4 floating 12V chargers) instead of a >> single 48V charger? > > not a whole lot. if you really want to balance them you should be using > 2V cells. you'd need 24, and 24 chargers.
Of course, getting 2V individual cells costs more and makes it harder to manage. I don't think you can "balance" batteries of different ages, conditions, etc. Rather, what I would like to avoid is having to replace entire stacks when one battery gets too far out of kilter. I am hoping that treating them individually might let me eek a bit more life out of not-yet-failed batteries
On 12/21/2016 9:04 AM, Jim Thompson wrote:
> On Wed, 21 Dec 2016 00:41:34 -0700, Don Y > <blockedofcourse@foo.invalid> wrote: > >> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here. >> >> "In theory", as all of the batteries see the same load, they should all >> degrade in lock step. >> >> "In practice", you expect some variations. >> >> How much can I potentially gain (life, capacity, etc.) if I design a >> "stacked charger" (e.g., 4 floating 12V chargers) instead of a >> single 48V charger? >> >> At the very least, it should facilitate replacing single batteries >> (instead of replacing the entire stack at the same time) -- ? > > Eons ago I designed a chip to charge-control multi-cell LiIon > batteries. It used shunting MOS FET's across each cell and a MUX'd > sensing system to "balance" the pack. Customer was California Micro > Devices (Tempe, near MicroRel, also one of my customers ;-). I'm not > sure if they even still exist.
Ha! Interesting approach -- tweek the load to make one charger "see" it as "consistent". I'd think it would be relatively easy to hack together a set of identical, floated 12V chargers (or 6V if I changed to a stack of 8). I'm just not sure how much benefit it will yield.
On Wed, 21 Dec 2016 09:04:22 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Wed, 21 Dec 2016 00:41:34 -0700, Don Y ><blockedofcourse@foo.invalid> wrote: > >>I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, here. >> >>"In theory", as all of the batteries see the same load, they should all >>degrade in lock step. >> >>"In practice", you expect some variations. >> >>How much can I potentially gain (life, capacity, etc.) if I design a >>"stacked charger" (e.g., 4 floating 12V chargers) instead of a >>single 48V charger? >> >>At the very least, it should facilitate replacing single batteries >>(instead of replacing the entire stack at the same time) -- ? > >Eons ago I designed a chip to charge-control multi-cell LiIon >batteries. It used shunting MOS FET's across each cell and a MUX'd >sensing system to "balance" the pack. Customer was California Micro >Devices (Tempe, near MicroRel, also one of my customers ;-). I'm not >sure if they even still exist. > > ...Jim Thompson
Looks like CMD was absorbed by ON Semi: <https://www.onsemi.com/PowerSolutions/content.do?id=16774> ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
On 12/21/2016 2:41 AM, Don Y wrote:
> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, > here. > > "In theory", as all of the batteries see the same load, they should all > degrade in lock step. > > "In practice", you expect some variations. > > How much can I potentially gain (life, capacity, etc.) if I design a > "stacked charger" (e.g., 4 floating 12V chargers) instead of a > single 48V charger? > > At the very least, it should facilitate replacing single batteries > (instead of replacing the entire stack at the same time) -- ?
The devil is in the details. You didn't say what type of battery. Makes a HUGE difference. -- Rick C
On 12/21/2016 3:48 AM, mike wrote:
> On 12/20/2016 11:41 PM, Don Y wrote: >> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, >> here. >> >> "In theory", as all of the batteries see the same load, they should all >> degrade in lock step. >> >> "In practice", you expect some variations. >> >> How much can I potentially gain (life, capacity, etc.) if I design a >> "stacked charger" (e.g., 4 floating 12V chargers) instead of a >> single 48V charger? >> >> At the very least, it should facilitate replacing single batteries >> (instead of replacing the entire stack at the same time) -- ? > If you want imbalance, replace single batteries. > > If you want balance, simplest is to build a voltage divider > and hook it to the batteries. Set them at 1% of the load and don't > worry about it. I pulled the 1% out of my ass. > Depends on charge current load current, duty factor...pick a number. > > This assumes they're charged frequently. If not, put the load on a connector > that you can plug in when you charge 'em. > > For batteries that are disconnected when charged, charge them in > parallel, with some current limiting light bulbs between 'em. > I did that with my bicycle light. 4 lithiums all brought out to > a connector. The bike runs them in series. Charged in parallel, > again with some current limits so they don't blow up if mismatched.
They are charged while "in use". Think of it as a 48V DC supply like CO battery. But, I don't have TPC's budget, nor staff! :-/
On Wed, 21 Dec 2016 10:45:38 +0000, Jasen Betts wrote:

> On 2016-12-21, Don Y <blockedofcourse@foo.invalid> wrote: >> I've several 48V, 9AHr battery stacks (4x12V@9AHr) that power a system, >> here. >> >> "In theory", as all of the batteries see the same load, they should all >> degrade in lock step. >> >> "In practice", you expect some variations. >> >> How much can I potentially gain (life, capacity, etc.) if I design a >> "stacked charger" (e.g., 4 floating 12V chargers) instead of a single >> 48V charger? > > not a whole lot. if you really want to balance them you should be using > 2V cells. you'd need 24, and 24 chargers.
IIRC the balancing needs of lead-acid cells are less than LiPo, so you can profit by charging them in units of 12V. -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com