T-Pain Is On A Boat: Re: [Electric Boats] Care and feeding of LiFePo batteries?

Wednesday, August 28, 2013

Re: [Electric Boats] Care and feeding of LiFePo batteries?

Dave,

Which cells are you using? I use Winston and set my low voltage alarm at 2.8 volts. I paid $1.10 per amp from Balqon.

Bob

From: Dave Steere <dcsteere@dcsteere.com>
To: electricboats@yahoogroups.com
Sent: Wednesday, August 28, 2013 7:37 PM
Subject: Re: [Electric Boats] Care and feeding of LiFePo batteries?

Li dead voltage (empty of charge) is around 2.6VDC per cell. Full charge is around 3.35V (after settling). You can charge them to as high as 3.7 (some references say 3.8V) without damaging, but they settle for a period of hours to around 3.35V per cell when full. I have no idea where you got 14V and 16V.

Likewise, if you discharge them to around 2.6V, they will very likely rejuvenate a bit in a matter of hours (or maybe a day) and you will need to again discharge to get the battery down to empty. You can buy lithium cells (LiFEPO4) for around $1.25 per Amp Hour. For example, my EV has 36 cells, each with 120 AH capacity. Thus my "full tank" voltage is around 121V. That means I have around 14KW continuous power available. My pack can deliver much higher current for very brief acceleration, on the order of 600-650 A, but you don't want to do that for more than a few seconds if you want to avoid having a dead duck for a battery pack. Total price for my pack is around $5400.

I don't know about trickle charging them, but I do know that just reaching full voltage alone is sometimes not entirely enough. A short time of constant current charging (after getting there with constant voltage charging) is a good idea when topping off, but one has to be very careful about over charging. I also can't give you first hand experience with number of cycles. My EV has been operational for about 14 mos. now and it's looking like around 5 or maybe 6 cells need replacing already. It's a big disappointment and I don't know yet why this has happened. I have never had any cell above about 3.8V or below about 2.3V.

Testing is still in progress, and I'm not real happy about replacing just a few batteries in the pack, knowing that I'll never achieve really good balance with some that came from completely different production lots. If you're not familiar with battery balancing, there is lots and lots of on line information about this. Some of it is pure BS and some is probably pretty accurate. One can either "bottom balance" or "top balance" and there's lots of controversy about which is the preferable method. But lithium batteries are much more sensitive about balancing than any form of lead acid, whether it be AGM, flooded, or gel. One more consideration (and this one is kind of controversial too) is the need for a BMS (battery management system). Lithium is much more in need of such a capability than lead acid. Lots of info about this on the various blogs and other web sites. I won't enter an opinion on these matters at the risk of stirring up a hornet's nest!

Dave

On Wed, Aug 28, 2013 at 3:33 PM, oak <oak_box@yahoo.com> wrote:

I was concerned to see the statement:
"Also they were trickle charged at all times, another no no".

Does that apply to LiFePo batteries?

Can anyone provide a summary of what the do's and don'ts are for these batteries?

This is probably asking wayyyy too much, but wouldn't it be neat if we could make a matrix of FLA, AGM, LiFePo, and show the various properties of each in a side by side arrangement?

I'd really like to see this for a 12V battery:

Property FLA AGM LiFePo
"dead" voltage 9V 9V 14V
"max" charge v 14V 14V 16V
Trickle ch maintain? Yes Yes no(?)
Average cycle life 200? 300? 1200?
Average cost (100Ah) $80 $180 $500+(?)
Maint high - water/acid none none
BMS no no strongly recommended
recovery when dead? yes yes no

Any other traits we can add to this, or other things to be aware of??

Note: by "recovery" - I heard that a Li battery is very good for holding it's voltage until it's dead. But once it's dead - it's DEAD. You can't wait for a few minutes and use it again lightly on the "recovery" like you can for FLA's.

John

From: Steve Dolan <sdolan@scannersllc.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Wednesday, August 28, 2013 7:10 AM
Subject: RE: [Electric Boats] Re: AGM Batteries?

Boeing's Dreamliner was an "international" plane built by contractors all over the world from many countries. As with anything like this it was a coordination nightmare. My understanding of this "event" was the problem was in the BMS system program not the batteries them selves. The system was spec'd to be at 100% charged at all times, something that has been determined to be on the ragged edge of theses batteries, you never want to overcharge them and this is what happened. They simply where over charged causing them to overheat. Also they where trickle charged at all times, another no no. I believe there were some issues with the BMS circuit boards as well on the bats. As for the chemistry, all it takes is someone to push hard from a major supplier from a country that planed to buy the Dreamliner who had an interest in sales first to talk someone into their product.

Steve in Solomons MD

But sport planes don't carry hundreds of passengers. If the amount of weight difference between LiFePO and LiCo is that critical, I am sure that it could be made up by creating carbon fiber drink carts instead of aluminum ones ;-) or reducing the baggage allowance per passenger accordingly.

The Dreamliner packs a total of about 5kwh of lithium batteries. Eric's 8kwh pack weighs 200lbs. So how much weight has been saved by using LiCo?

Using your own 25% lighter than LiFePO figure, that would amount to a whopping 50lbs. The maximum take-off weight of the Dreamliner is about 500,000lbs. So we're talking a 1% of 1% weight savings by using LiCo. Bravo Boeing.