 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Electronics : Charging a Li-ion battery pack
| Author | Message | ||||
| amalething Newbie  Joined: 24/03/2015 Location: United KingdomPosts: 4 |
Hello, I'm replacing a 7.2v Varta 250mAh NiMH backup battery pack from an old device that has an 8.4v constant charge feature. I am intending to replace the battery with 2pcs of ICR14500 (total 7.2v) with protection chips built in. It would be impossible (for me) to replace the charge function of the device. My question is, regardless of performance, life, suitability, etc. I just need to know will the Li-ion battery explode or just fail (i.e. refuse charge or lose life). Appreciate your input. Darren |
||||
| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
The really long answer to this one is found here: House banks This is page 175 out of about 300. It has taken several weeks to get this far! BatteryUniveristy has a good summary page and even Wikipedia Lithium ion is quite useful. One of the important factors appear to be which type of lithium ion battery is it. The thread I have linked to is focussed mainly on LiFePO4. The solution to your "explode" comment is: not to over-charge. For LiFePO4 the recommendation is to limit the charge to achieve 80-90% of "full" capacity and also to terminate charge, ie don't float. For maximum life the recommendation is to only put as much charge in as you need, ie for a days operation then replace it. They are suppose to be quite efficient. Ideally, you want to cycle between 20 to 50% AND 80% on the high side for best life. So, the short answer using your current charger is "possibly explode" or over-heat, you really need to know where the "upper knee" kicks in. In other words 100% SOC. 7.2V is nominal, what voltage corresponds to 100% SOC? The charging regime also seems quite dependent on how you are going to use the battery, ie RC control, house banks, EV, etc. |
||||
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
It will depend on how your existing NiMH charger works. if it charges at 10% of the old batteries(25Mah) it will work but take 28+ hours to fully charge the new cells and could overcharge the li ions if it does not switch off. if it has a temperature controlled charge algorithm, you might be screwed,it may not start not sure on that. NiMH chargers are a pain in the neck. LI Ion chargers are designed to cut out at a certain current level (quite often 0.1 amps), if it keeps charging at 8.4 volts once the batteries are full it will cook the batteries. it may work if it cuts back to a lower float voltage or you run the batteries down to the low volt cut off and use a timer so they dont overcharge. Wiring in a 2 cell wall wart would be the best bet if that is possible. this sort of thing is about the right size May be a little big? even better would be to make them removable and charge them individually (not in series) in a cradle that would avoid any problems keeping them balanced. 1600 or 2000 Mah capacity would extend the time between charges and make it less painful to use a cradle. hope that helps a little. cheers I'm confused, no wait... maybe I'm not... |
||||
| amalething Newbie Joined: 24/03/2015 Location: United KingdomPosts: 4 |
Hi Chaps, This is really helpful advise. Finding information about lithium based cells is difficult, and I'm more than welcome any suggestions for references. Possibly I'm in the wrong market, which the wrong education. We're a small cell pack manufacturer for simple NiMH/NiCD packs. I'm trying to branch into lithium cells, however there seems to be a vast gap of available knowledge between advise for those wishing to power torches >> and to the extreme, discussing the chemistry side. So when it comes to building custom Li-ion packs and charging them, the answer is always to put them in a single cell charger. In every case, our cell packs are placed inside a OEM device, charged on a float, and inaccessible. So with that background, the simple answer is, don't use lithium. But would anyone be able to suggest a more tolerant lithium cell? I'll look into LiFePO4 as an option. How might I find the float point of the device? Thank you for your comments, they're extremely helpful. Darren |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I am guessing you want to look for the voltage where putting in more current does not raise the terminal voltage. Possibly coincident with the generation of heat. If you do not have the possibly of changing the charger behaviour I suspect you are confined to NiMH. There are a number of Li-ion technologies, maybe somebody supplies or is working on one that is more tolerant of being floated. |
||||
| amalething Newbie Joined: 24/03/2015 Location: United KingdomPosts: 4 |
Thanks davef. Is it the NiMH cells or charger that detects the heat? But more importantly, you say when the cell generates heat, (with NiMH and NiCD) it means its fully-charged. However with Li-ion, heat means its cooked, and you've burnt it round the edges. I spoke to our supplier this morning. They confirmed that if one was to use constant voltage at 50mA, the cell would be ok. Where our cells are protected, this prevents over charge. This is confirmed in Electronics Weekly, when I searched for 'floating Li-ion' as you suggested. "Any current forced into a fully-charged Li-ion damages it. At minimum this reduces cell life. At the extreme maximum some unprotected cells have been known to burst and catch fire. Protection against failed voltage or current regulation should always be present in a Li-ion or lithium polymer charging scheme." There are two ways to read this, but if I'm right this states with protection, current forced into a fully-charged Li-ion battery would not damage or reduce the cell life. My next question put to my supplier is regarding the protection; how much current can the protection handle. I'm enjoying learning about the chemical construction and limitations of cell technologies. Something I didn't particularly need before, but should have learned non the less. |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Not the way I read it. I read two statements, one should not float for max or good lifetime AND as a back-up ensure your charger design does not fail so as to place "unregulated maximum current or fault current" to the battery. 50mA is only about 0.5Watt being dissipated. If they have some test results to back up what the reduction in cell life would be at 50mA, then you could make a decision. |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
But, depending on the design parameters built-in to your charger it sounds like you could still be stuffing current into the cell. Also, is seems that without exception LiFePO4 systems have both a HVC (high voltage cutout) and a LVC (low voltage cutout) as they can also be damaged by letting the terminal voltage get too low. Keyword here is BMS (battery management system). I think you will find that, for example, laptop Li-ion battery systems have some fairly complex cell management circuitry. |
||||
| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi Amalething, what i did to get myself up to speed was look at the RC market, those guys are all over series parallel pack charging. ARK- Revolectrix Cellpro chargers the hobby charger is a great tool to easily see what combinations work and how it is all done at the ragged edge of battery use. The torch I use the most is a fenix TK15 with a single 18650 3100MaH Li-ion but the one that I find the most useful for work is one of their headlamp range, the choice of battery is 2x AA or NiMH, NOT 14500 Li-ion (they list it as banned for this use). It works for me, I work long hours in the dark for a good part of the year, 2600Mah NiMH's last a hell of a long time on the lower power settings. I have only been caught with failing light once, I had thought about swapping the cells before I left home because I hadn't changed them over for a couple of weeks, I had an emergency to deal with on the side of the road and I needed all the light it could give for more than an hour. http://www.fenix-store.com/aa-ni-mh-rechargeable-kit/ I think it is still a cost effective and practical solution to use NiMH but it is very hard to design something that is as effective as cradle charging for high power rated batteries and keep the price down. There is always a compromise and it is usually battery life from poor charging. yahoo I'm confused, no wait... maybe I'm not... |
||||
| amalething Newbie Joined: 24/03/2015 Location: United KingdomPosts: 4 |
I've heard of people putting the 14500 into torches. Increased voltage by 3x, but reduced capacity cells. I'd like to observe for myself, although I imagine one would go through bulbs quick, and I hear the torches get pretty hot! You're right, I hastily read it as the protection being in the cell, not the charging scheme. But you think the LiFePO4 might be an answer. I'll venture into area with our supplier. |
||||
| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
I am not suggesting it is a solution. I have only done quite a bit of reading with a view to replacing lead-acid in an off-grid house bank. I am no battery expert! Good luck. |
||||
| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
the output of the new LED's can be truly massive http://www.torchworld.com.au/catalog/index.php?main_page=product_info&cPath=96&products_id=820 I'm confused, no wait... maybe I'm not... |
||||
 Print this page |
