## Re: [RoboMower] Re: LiFePO4

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• Hi Wofbob,   Yes, it s impossible to make 24V20AH with only 20 cells of 3.3V2.3AH. Divided 24V by 3.3V, 7 or 8 strings of cells should be connected in serial,
Message 1 of 18 , Feb 9, 2009
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Hi Wofbob,

Yes, it's impossible to make 24V20AH with only 20 cells of 3.3V2.3AH. Divided 24V by 3.3V, 7 or 8 strings of cells should be connected in serial, and divided 20Ah by 2.3Ah, 8 or 9 cells should be connected in parallel, so, at leat 7*8=56 cells will be needed.

Best regards,
Jerry

--- 09年2月10日，周二, wolfbob <wolfbob@...> 写道：

发件人: wolfbob <wolfbob@...>
主题: Re: [RoboMower] Re: LiFePO4
收件人: RoboMower@yahoogroups.com
日期: 2009,210,周二,7:36上午

I am having trouble with the math. The A123 is 3.3V at 2.3AH How do I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH or 33V at 4.6AH. I would think you would need 8 banks of 8 cells.

WBob

----- Original Message -----
From: Melissa
To: RoboMower@yahoogrou ps.com
Sent: Monday, February 09, 2009 7:53 AM
Subject: [RoboMower] Re: LiFePO4

> I think these are called A123 cells here in the US.

We're using these in our Gen 1 Automowers. We have 2 banks of 10, and
they fit right in the original battery case. The battery life between
charges is excellent. We've only used them one summer, so we don't
know how long the life span will be.

Melissa

[Non-text portions of this message have been removed]

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• With regard to the comparison between small cell or big prismatic cell for battery pack, pls read the article at
Message 2 of 18 , Feb 9, 2009
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With regard to the comparison between small cell or big prismatic cell for battery pack, pls read the article at http://liionbms.com/php/wp_lots_small_cells.php%c2%a0. In addition, up to now, I do not find any car makers make their electric cars with large format cells. Almost all of them use small cells with capacity of less than 3Ah.

Of course, there're many personal DIY fans who make their own evs by using large format cells with capacity of more than 100Ah/cell. The major reason is that the connection of the cells are much easier for them to choose large format cells.

With regard to the calendar life, I think LiFePO4 is good. We had a test that some cells charged full were left there unused from 2004 and they still had capacity of over 70% in 2008.

The problems for LiFePO4 are cost and weight. For RoboMower application, no need to worry about the battery's performance.

Best regards,
Jerry

--- 09年2月10日，周二, Danny Miller <dannym@...> 写道：

发件人: Danny Miller <dannym@...>
主题: Re: [RoboMower] RE: LiFePO4
收件人: RoboMower@yahoogroups.com
日期: 2009,210,周二,3:35上午

That may or may not be true.
LiFePO4 do have a significant voltage drop. The question is, how many
can we put in series before the peak, full-charge voltage is
unacceptable to the mower? Unless there's a series regulator (kind of a
bad idea) we mainly have this question of whether it can be a 7-cell or
an 8-cell pack. My initial survey of the problem seems to say 7-cell is
the limit, which makes the "average" voltage across the entire run will
actually be near what a lead-acid has at the end.

I may have been wrong though. I think I was looking at Li-Ion curves
which is wrong. It looks like LiFePo4 is substantially different and
the "24V" packs DO use 8-cells and have a lower peak-charge voltage.

The \$300 sellers on eBay are being referred to as "duct tape" sellers,
because their packs are all duct taped together. Apparently they
assemble these out of a massive number of smaller cylindrical cells as
opposed to a few larger prismatic cells. This solution tends to be low
reliability since the failure of any one cell can result in the pack
being unusable. Thus the cost-effectiveness of a gajillion cycles at
twice the price of a lead-acid may not happen. Well, they're not spoken
of in a favorable tone anyways.

It looks like YESA is the company making "proper" cells and packs.
AFAIK the cells are large prismatics not a bunch of smaller cylindricals
put in parallel. I could be wrong about what's under that "duct tape"
in those eBay packs, some may be putting YESA cells together for all I
know. I haven't tried to buy one.

Danny

Austin Morgan wrote:
> Also the power curve of the LI batteries means I
> would constantly get a better mow because with the Lead Acid batteries the
> mower does a much better job a the beginning then at the end.
>
> Austin

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• You d put n cells in parallel and then put those in series. OK, looking at more actual data, the A123 is 3.3v nominal, and 3.6v max at full charge. So 8x in
Message 3 of 18 , Feb 9, 2009
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You'd put "n" cells in parallel and then put those in series.

OK, looking at more actual data, the A123 is 3.3v nominal, and 3.6v max
at full charge. So 8x in series is 28.8v which the Robomower can take.
It will be 26.4v for the bulk of the discharge curve and that's slightly
better than the 24v nominal of the lead acid packs.

Anyhow, the 24V20AH "bike" packs discussed are LiFePO4 but NOT A123
brand. I don't know how many cells they put in parallel into each bank,
or the capacity of each cell, but apparently these are 8x parallel banks
in series. The voltage should be similar.

So to get something comparable in runtime to the 20AH lead-acid, you'd
need 8x of the 2.3AH A123's in parallel, or 64x total. At the eBay
price of \$8/cell (which seems to be the best deal right now) that's a
\$512 pack. The duct tape "bike packs" are \$300, but they're hardly the
proven quality of A123, and they're not rated for extremely fast charge
currents like A123. Which you may not care about, I mean you'd need a
bitchin powerful charger and balancing gear to get a >20A charge rate
which is a whole 'nother ball of wax.

I do not know if the Robomower's pack geometry would allow for 64x A123
cells to fit. I would think so though, they're not all that big.
Cooling can be a problem in densely packed configurations.

I was surprised to see someone just started listing 10x lots of A123
2.3AH cells at \$40. That is half what they've been running, but I think
the demand is so high the final selling price will be ~\$8 again.
Hundreds of people would jump at volume supply of \$4 cells. There are
no Completed Auctions on the 10x lots yet to gauge the price.

Danny

wolfbob wrote:
> I am having trouble with the math. The A123 is 3.3V at 2.3AH How do I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH or 33V at 4.6AH. I would think you would need 8 banks of 8 cells.
>
> WBob
>
>
> ----- Original Message -----
> From: Melissa
> To: RoboMower@yahoogroups.com
> Sent: Monday, February 09, 2009 7:53 AM
> Subject: [RoboMower] Re: LiFePO4
>
>
> > I think these are called A123 cells here in the US.
>
> We're using these in our Gen 1 Automowers. We have 2 banks of 10, and
> they fit right in the original battery case. The battery life between
> charges is excellent. We've only used them one summer, so we don't
> know how long the life span will be.
>
> Melissa
>
>
• WBob the automowers use 18v batteries not 24v you need to recalculate. ... I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH or 33V at
Message 4 of 18 , Feb 9, 2009
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WBob the automowers use 18v batteries not 24v you need to
recalculate.
--- In RoboMower@yahoogroups.com, "wolfbob" <wolfbob@...> wrote:
>
> I am having trouble with the math. The A123 is 3.3V at 2.3AH How do
I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH
or 33V at 4.6AH. I would think you would need 8 banks of 8 cells.
>
> WBob
>
>
> ----- Original Message -----
> From: Melissa
> To: RoboMower@yahoogroups.com
> Sent: Monday, February 09, 2009 7:53 AM
> Subject: [RoboMower] Re: LiFePO4
>
>
> > I think these are called A123 cells here in the US.
>
> We're using these in our Gen 1 Automowers. We have 2 banks of 10,
and
> they fit right in the original battery case. The battery life
between
> charges is excellent. We've only used them one summer, so we
don't
> know how long the life span will be.
>
> Melissa
>
>
>
>
> [Non-text portions of this message have been removed]
>
• A123 s cell is also of LiFePO4 chemistry. It s known to all the A123 s cell has very good performance in high power discharge and charge. But I m not sure
Message 5 of 18 , Feb 9, 2009
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A123's cell is also of LiFePO4 chemistry. It's known to all the A123's cell has very good performance in high power discharge and charge. But I'm not sure whether it is the best one.

I don't think A123's cell is necessary for RoboMower application. Generall high capacity cell will be OK.

Best regards,
Jerry

--- 09年2月10日，周二, Danny Miller <dannym@...> 写道：

发件人: Danny Miller <dannym@...>
主题: Re: [RoboMower] Re: LiFePO4
收件人: RoboMower@yahoogroups.com
日期: 2009,210,周二,10:35上午

You'd put "n" cells in parallel and then put those in series.

OK, looking at more actual data, the A123 is 3.3v nominal, and 3.6v max
at full charge. So 8x in series is 28.8v which the Robomower can take.
It will be 26.4v for the bulk of the discharge curve and that's slightly
better than the 24v nominal of the lead acid packs.

Anyhow, the 24V20AH "bike" packs discussed are LiFePO4 but NOT A123
brand. I don't know how many cells they put in parallel into each bank,
or the capacity of each cell, but apparently these are 8x parallel banks
in series. The voltage should be similar.

So to get something comparable in runtime to the 20AH lead-acid, you'd
need 8x of the 2.3AH A123's in parallel, or 64x total. At the eBay
price of \$8/cell (which seems to be the best deal right now) that's a
\$512 pack. The duct tape "bike packs" are \$300, but they're hardly the
proven quality of A123, and they're not rated for extremely fast charge
currents like A123. Which you may not care about, I mean you'd need a
bitchin powerful charger and balancing gear to get a >20A charge rate
which is a whole 'nother ball of wax.

I do not know if the Robomower's pack geometry would allow for 64x A123
cells to fit. I would think so though, they're not all that big.
Cooling can be a problem in densely packed configurations.

I was surprised to see someone just started listing 10x lots of A123
2.3AH cells at \$40. That is half what they've been running, but I think
the demand is so high the final selling price will be ~\$8 again.
Hundreds of people would jump at volume supply of \$4 cells. There are
no Completed Auctions on the 10x lots yet to gauge the price.

Danny

wolfbob wrote:
> I am having trouble with the math. The A123 is 3.3V at 2.3AH How do I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH or 33V at 4.6AH. I would think you would need 8 banks of 8 cells.
>
> WBob
>
>
> ----- Original Message -----
> From: Melissa
> To: RoboMower@yahoogrou ps.com
> Sent: Monday, February 09, 2009 7:53 AM
> Subject: [RoboMower] Re: LiFePO4
>
>
> > I think these are called A123 cells here in the US.
>
> We're using these in our Gen 1 Automowers. We have 2 banks of 10, and
> they fit right in the original battery case. The battery life between
> charges is excellent. We've only used them one summer, so we don't
> know how long the life span will be.
>
> Melissa
>
>

___________________________________________________________
好玩贺卡等你发，邮箱贺卡全新上线！
http://card.mail.cn.yahoo.com/

[Non-text portions of this message have been removed]
• OK..I am still lost... Two banks of ten give 33V at 4.6AH still not 18V either. If you are going to arrange 20 cells for 18V then I would think you would have
Message 6 of 18 , Feb 10, 2009
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OK..I am still lost...
Two banks of ten give 33V at 4.6AH still not 18V either. If you are going to arrange 20 cells for 18V then I would think you would have 5x4 for 16.5v & 9.2AH. Better to have 24 cells at 6x4 for 20V at 9.2AH.

I would think if these cells are going to be popular, then we should consider electronic designs in multiples of 3.3 volts, not the 1.5 volts of "old" batteries.

Once these "little" cells get popular in the automotive market, the price will plummet. A Hybrid car will use hundreds of these cells each and an electric car several hundred. I expect to see them under a dollar in 5-7 years so the idea of stuffing 50 of them into your mower is not too ridiculous.

WBob

From: bkelley9401
To: RoboMower@yahoogroups.com
Sent: Monday, February 09, 2009 6:45 PM
Subject: [RoboMower] Re: LiFePO4

WBob the automowers use 18v batteries not 24v you need to
recalculate.
--- In RoboMower@yahoogroups.com, "wolfbob" <wolfbob@...> wrote:
>
> I am having trouble with the math. The A123 is 3.3V at 2.3AH How do
I get to 24V and 20AH with 2 banks of 10?? I either get 6.6V at 23AH
or 33V at 4.6AH. I would think you would need 8 banks of 8 cells.
>
> WBob
>
>
> ----- Original Message -----
> From: Melissa
> To: RoboMower@yahoogroups.com
> Sent: Monday, February 09, 2009 7:53 AM
> Subject: [RoboMower] Re: LiFePO4
>
>
> > I think these are called A123 cells here in the US.
>
> We're using these in our Gen 1 Automowers. We have 2 banks of 10,
and
> they fit right in the original battery case. The battery life
between
> charges is excellent. We've only used them one summer, so we
don't
> know how long the life span will be.
>
> Melissa
>
>
>
>
> [Non-text portions of this message have been removed]
>

[Non-text portions of this message have been removed]
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