Batteries and New Battery Technologies

Reversal

Hi Porta,

Absolute no idee what will happen. I try to stay away from reversing any battery, but indeed it can happen. Will see what happens with a couple of small lithium batteries, if one cell get reversed. I dare don't try that with a 50Ah battery untill we know what the consequence is.

Are you only worried during the charging process or at any given time?

BertKu

 

A BMS that is working properly should prevent any cell from dropping below it's minimum voltage, let alone reversing. The BMS systems I have all have some form of cell level LVC, one actually disconnects the battery if any cell drops below a safe minimum voltage, the other sends an ebrake signal to the controller to turn it off.

These BMS units also control cell voltage on charge, both limit individual cells to the max allowable charge voltage using shunts together with charge control switches. These effectively give the CC/CV charge profile that lithium chemistry cells need.

Running lithium cells without some form of effective BMS is both hazardous and likely to result in a very short operating life. I know that the RC model people tend to not use discharge control BMS units, but they do tend to accept a high cell failure rate as a consequence.

With the high initial purchase cost of batteries, the added expense of a decent BMS seems to be a small price to pay to protect an investment. It's possible to buy a simple, cell-level discharge warning and indicating system (with data logging) that will monitor up to 8 series cell packs for under $40, (see here: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=9282&Product_Name=Cell-Log_Cell_Voltage_Monitor_2-8S_Lipo for example). This very cheap unit has an alarm output that can be used to switch a relay or turn off a controller, so would provide adequate discharge protection for a low voltage system. I can't see any good reason why several couldn't be used to monitor bigger battery packs, although the alarm outputs would need to be OR'd together using optoisolators.

Jeremy

 

I could not get small re-chargable batteries in this area on short notice, thus I have send a fax to the manufacturer.

I have had customers who saw chance to put 6 Volt batteries on 12 Volt chargers, reversing batteries and blew my charger, anything is possible. It is a very good question: what happens if one cell is reversed and the charger is put on 50 Ampere charging current. The MBS maybe should warn beforehand?

BertKu

 

For my projects, I prefer to use Li snap in paks that come from and belong with a particular cordless appliance, say cordless drills. I simply attach my own appliance via the mating spade terminals, then remove the mates to recharge. My reasons for using these packs are: lower cost due to price wars and universal availablity, generous replacement warranties, bells and whistles -like a fuel guages, and of course, saving time and parts costs required for assembling your own pack and charger.

When I began this several years ago the BMS was found in the snap on battery module itself. Now, sometimes the BMS will be in the driven tool or the charger- instead of the battery pack. Maybe you can still use the battery module itself in projects by watching the gas guage to avoid damaging discharge? Unless there are individual cell shutoffs inside the separate pak, cell reversal is possible.

Porta

 

Clever

Hi Porta,

That is indeed a clever idee. I will shop around and see which tools has Li packs and what the cost is. It depends on what type of Lithium batteries they are using, the upper charging voltage ( > 2.8 Volt) and lower Voltage for discharged < 2.3 Volt may differ, as in the case for the Li-titanate.
I do not believe that the actual charging current is very important, because the Altair curve is from 50 to 500 Ampere and I assume that one can charge
basically any current, as long it is less than 500 Ampere. It is the temperature/max charging voltage and lowest discharged voltage per cell what is important.

BertKu

 

Here is what is currently available with similar nano type technology of the altair:
http://www.amazon.com/gp/product/B0...ils?ie=UTF8&me=&qid=1216536567&sr=1-2&seller=

May be too small for your project, unless you can go to higher voltages. Nano is quite expensive per watt hour, and I expect the altair would be considerably more expensive if they hold to the stated specks.

The lowest cost per watt hour I have found is
http://www.amazon.com/gp/product/B000JTBPF6
I wouldn't think these are nano technology based at that price, though.

A web search might get you better prices, but this gives an idea. Salvaged packs may be available for rebuilding yourself at considerably lower prices.

Porta

 

Hi Porta,

Thank you for your info. Great stuff. However, it is a little expensive for me to blow a brand new pack by reversing one of the cells. I will shop around locally and see whether I can get some old ones from the repair shops, if they have any. All what I need is 3 or 4 cells and reverse one and put a few ampere on them. Take the nessecary pre-caution and off we go.

Anybody have any experience with a NEAR discharged Lithium battery and left it for a few years?

I have searched the web, but nowhere any information on a cell reversed. It will one day happen, specially if one is on a boat, it becomes dark, don't feel well, you have a problem with one of the cells, which needs to be replaced, you struggling and acidental you make a mistake and we need to know what is the consequence. With a battery pack it is all controlled, but with some 20 cells or more of 1.6 Kg it is a different story.

BertKu

 

If 10 Lithium cells are put in serial and one of the cell is reversed, one will not have 30 volt but 30 - 2 x 3 = 24 Volt. Although at reduces voltage level, I could not detect any heating up nor damage to the cells. I could connect a load and the load was working at reduced power. Like Jeremy in a previous comment said, a good BMS should give an alarm.

I was already wondering why you cannot find single re-chargable batteries in the shop, other than complete sealed packs with multiple cells and a BMS.

I bought a few inexpensive cellphone (mobile phone) Lithium batteries and did some experiments, during charging with a cell reversed, very quickly one could see why not. If I had carried on and charged it longer it would have exploded due to the internal heat generated. The metal started to buckle.

Having a number of 50Ah batteries in series and no good BMS is suicide. One cannot just ask anybody to charge those batteries without some experience and trust that the manufacturers will come out with battery terminals wereby one cannot easy reverse terminals.

I can't wait for Super Capacitors to come out of the production lines. You ship them with a plane, without concern. No danger, you should be able to reverse them and they last a million deep discharges and charges.

BertKu

 

Reversal

Hi Gustav,

Thank you for your e-mail. Yes indeed, when you are a visitor to Boat-design.net, you cannot reply unless you are registered. My apology, I did not explain it clearly. If you reverse a cell of 3.0 Volt, The overall voltage of the system is not reduced by the voltage of that cell only, but also by the cell which compensate for this reversed cell. Thus 30 Volt minus 3.0 volt minus 3.0 volt = 24 Volt. Should you thus reverse 2 cells in a 10 cell system, you loose thus not just 2 x 3.0 Volt, but 2 x 2 x 3.0 Volt = 12 Volt and your resulting terminal voltage will be 18 Volt.

BertKu.

 

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sure i belive you yet i dont like to hand in my phone number in advance
was there anything important in peticular i might like to know?
( like f.e. the $ 5k price on a lithium battery )

 

Yipster,
I am only registered to the email.
Every week I scan the titles and just goa and read if there is something interesting.
I gave my number, but nobody has rung me yet. You can put there any number (123456789) and you will still get the weekly newsletter...

 

Seems that you are writing that a reversed cell within a Li battery acts differently than other chemistry types. In a Li battery a reversed cell is not just a dead cell that conducts, but that the reversed cell generates an opposing voltage to the rest of the pack while conducting. Wonder how long this behavior for the reversed cell continues before it just dies. At the point the reversed cell dies, then maybe it just conducts without an opposing voltage or causes an open circuit?

Your point about charging with a reversed cell in a Li battery is well taken. Remaining unreversed cells would be overcharged with possible fire consequences is what I understand... Not good.

Porta

 

Hi Porta,

It actual seems to apply to all batteries. I also tested a SLAB (Sealed Lead Acid) and some 1,2 Volt NiMH in the same way. They also behave in the same manner. I tested 4 x 6 Volt SLAB batteries with one reversed and also had only 12 Volt. When I connected a 12 Volt 50 watt globe to it, it just glowed the same way as off on a 12 Volt battery. I am not a chemist, and need my batteries and cannot just blow them. I cannot answer your question. Maybe somebody else can comment?

BertKu

 

You see Porta,

What I am interested in is, is the danger I expose myself by doing something stupid. Everybody does something he will feel sorry about. In my case, it will take a long time before we will see Super Capacitor batteries. Thus in the meantime I need to use most likely Lithium batteries. If one charges a battery, normally one charges the battery with a higher current than the day to day usage current. Thus I needed to know what the consequence is, and what kind of action I need to take to prevent myself /and/or my family to expose myself to such danger. I personally think that Lithium is safe, provided a good BMS is connected and double check the replacement, if batteries are replaced. That is all. But a reversed battery must be picked up by a good BMS and therefore I have no longer an interest in other side effects of reversed batteries. I may have disappointed you, for not be able to answer your question. I am so sorry for that.

BertKu