Converting 2200VA UPS from SLA to Li based batteries.  - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: March 03, 2016, 02:01:55 am
content: A friend of mine asked me the other day if it was possible tosafelyconvert a rack mount commercial UPS from SLA to a Li based chemistry. I did not have an answer since I have no idea how the charging circuit works or what input voltage range the units typically have.The goal is to save weight since the unit is always traveling (on land). The lead acid batteries are very heavy.Has anyone done a conversion? I should qualify that by saying, has anyone done a safe conversion? Probably looking at LiFe batteries. I have a solid skill set in power electronics, but never tried this. All of my design is my own - not having to modify or interface with others.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 03, 2016, 02:35:21 am
content: LiFePO4 is nearly a drop in replacement for lead acid.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 03, 2016, 02:42:22 am
content: Quote from: NiHaoMike on March 03, 2016, 02:35:21 amWhen you say 'nearly', does that mean it still requires some delicate control to keep the fire away?

author: TheSteve
author_rank: Unknown
timestamp: March 03, 2016, 06:29:33 am
content: I'd start by testing to find the maximum voltage the UPS will charge batteries too and at what voltage it shut down to protect the batteries. Once you have the min/max voltage you can then see if there is a suitable lithium chemistry that will match the stock parameters. If there is then you'd just want to add some sort of cell balance protection.

author: Nebulex
author_rank: Contributor
timestamp: March 03, 2016, 04:49:31 pm
content: We use some LiFePo4 cells and charge them with a maximum voltage of 3.55V per cell ( for life time purpose) even if the datasheet says you can charge them to 3.6V. And do not forget the cell balancing circuit that has to control also the final discharge voltage of about 2.6V (we stop at 2.85V) according to the datasheet.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 03, 2016, 05:28:01 pm
content: Tell us about the original lead acid voltage.I'd say that it's rather OK to convert a 24V SLA UPS to 7s li-ion using energy dense modern NCA cells, as long as they are from proper manufacturers (i.e., Sony, Panasonic/Sanyo, Samsung, LG), without any kind of BMS or balancer. Other experts may have differing opinion, and I can't guarantee the safety of this, but I can't guarantee anything anyway in a forum post. But converting a UPS to li-ion without any BMS is something I would do (and in fact I'm currently considering, having a 1400VA 24V SLA UPS right here with a dead battery).Anyway, BOSCH goes up to 6s with no cell level BMS whatsoever on NCA cells. 7s is not far from this. 2s without cell level monitoring/balancing is industry standard maximum which practically everyone agrees with. After that, opinions vary.Inferior LFP chemistry, as suggested above, is not needed if the UPS is a 24V system, NCA or LCO (min-avg-max 3.0-3.65-4.2V) works well in 7s configuration. For a 12V system, 4s LFP (3.0-3.25-3.5V) is the only choice; with NCA, 3s is too little, 4s is too much.Lead acid UPS already has proper pack-level protection for both charge and discharge. Charging current is also probably low enough (less than 0.5C). It's not a drop-in replacement, as voltage limits may need some adjustment. Most probably, the lead acid limits are too tight, so it would work safely, but only get you about 60-70% of the full capacity, which would still provide much better Wh/kg on NCA than getting 100% from LFP ever would.For 12V systems, 4s LFPs are readily sold as drop-in replacement modules today! That's the easy solution, but LFP energy density is not extraordinary (ca. 120 Wh/kg vs. 250 Wh/kg of NCA), even though it's still many times better than the lead acid, especially at high discharge rates usually seen in UPS systems (ca. 25 Wh/kg).

author: dwpatter53
author_rank: Contributor
timestamp: March 14, 2016, 07:27:53 am
content: For Li batteries you need to control the voltage carefully to not overcharge the battery.Heat and current to each cell has to be limited to prevent a meltdown.Here is some good references;http://batteryuniversity.com/learn/article/charging_lithium_ion_batterieshttp://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteriesCheers

author: rx8pilot
author_rank: Super Contributor
timestamp: March 14, 2016, 05:47:08 pm
content: Quote from: dwpatter53 on March 14, 2016, 07:27:53 amThank you,It seems unrealistic to convert to any Li chemistry without a complete replacement of the charging sub-section of the UPS. At least if it is going to be done safely.Looking for other options since this concept appears to be a near do-over on the design of a UPS and I am not in the UPS business.