Replacing APC ES-550 Lead-Acid Battery with 18650 Pack – Is It Safe? - Page 1
Forum section: electronics / general

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 02:56:16 pm
content: I bought a 3x7 18650 Li-ion battery holder (link below), and it hasn’t arrived yet. My APC ES-550 normally uses a single 12V 9Ah lead-acid battery — I’m wondering, can I safely replace it with a pack built from 18650 cells?I’m not an expert on Li-ion cells, but I do have some tools (like a spot welder, an RC3563, etc.). Still, repairing something is very different from experimenting with new builds or changes. I'm mainly interested in understanding the UPS electronic design and chemical safety aspects before I try anything.https://www.amazon.ca/Battery-Storage-Uninterrupted-Special-Plastic/dp/B09N2FWSFJ

author: Kjelt
author_rank: Super Contributor
timestamp: September 19, 2025, 03:00:47 pm
content: Are you going to modify/replace the charging circuit with a LiIon charger and BMS with cell balancing?If not: NO! You will burn down your room.Lead Acid is Constant Voltage charging.LiIon is combined Current/Voltage charging with load balancing and End Voltage stopping.Continuously charging it with a LeadAcid charger will overheat and the cells will catch fire. You will get something like this:

author: wraper
author_rank: Unknown
timestamp: September 19, 2025, 03:01:08 pm
content: You can do that as a time bomb for your enemy.

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 03:15:30 pm
content: Quote from: Kjelt on September 19, 2025, 03:00:47 pmSorry, I forgot to mention that. Yes, I’m also going to use a 3C40A BMS.

author: tunk
author_rank: Super Contributor
timestamp: September 19, 2025, 03:27:26 pm
content: As Kjelt pointed out, in addition to the BMS you need a Li-Ion charging circuit.Edit: Also, you need to check that your 18650s can deliver enough current.

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 03:55:37 pm
content: The box comes as a complete set without batterys. That means, in addition to the items listed below, it should also include the BMS charger.

author: tunk
author_rank: Super Contributor
timestamp: September 19, 2025, 04:28:00 pm
content: Usually BMSs doesnotcome with a charging circuit.Most of the BMSs provide protection for this:- over-charge- over-discharge- over-currentLooks like this in addition has some kind of balancing.

author: squadchannel
author_rank: Frequent Contributor
timestamp: September 19, 2025, 04:28:42 pm
content: why replace to lithium? if you want to extend backup time, you should use a model with a larger capacity.even if backup time were extended, it's unclear whether the other components could hold up for that time. small UPS like this are designed to provide backup for minutes to tens of minutes.they are not designed to provide backup for hours.and, UPS must reliably provide backup during emergencies. it would be difficult with a diy battery.UPS is not a portable battery!just $20, you could buy a lead battery.

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 04:31:39 pm
content: Quote from: tunk on September 19, 2025, 03:27:26 pmI will use brand-new, high-quality 2800mAh 18650 batteries.

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 04:42:06 pm
content: Quote from: squadchannel on September 19, 2025, 04:28:42 pmI already have a 1 kVA online UPS. My goal with this one is to run simple devices, like a fiber modem or other small gadgets.I don’t remember if the APC behaves like this, but I recall that many small UPS units have software that shuts off the output after a certain period (maybe around 10 minutes), even if the battery is fully charged.I have an APC ES-550. I want to experiment with it to both put it to use (recycling) and learn something new. Plus, with 21 × 2700 mAh cells, it could offer decent capacity. I’m just tinkering.

author: wraper
author_rank: Unknown
timestamp: September 19, 2025, 06:49:32 pm
content: Note that usual (not special high current cells) 18650 cells have about 1C maximum discharge current. Which will be like 15-20A with 7 cells in parallel. But this UPS will draw like 60A under max load. Also you need a different charging circuit, not just BMS as UPS will try to charge them to about 4.8V per cell with 3S battery. 4S LFP is the only thing that might work fine with not modifying UPS itself.

author: Sercan
author_rank: Contributor
timestamp: September 19, 2025, 08:24:41 pm
content: I already bought this box and also got 30 18650 batteries. Even if I don’t use them for this, I can still make use of them.As far as I can tell, if I drop the UPS power strip idea and connect this box directly to the modem adapter (paralel), it should have enough power to keep the modem running for hours, maybe even a couple of days, during a power cut.Thanks for your help, folks!I’d be happy to hear any other ideas you might have.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 20, 2025, 04:11:19 am
content: If you just need to power a fiber to copper converter (technically not a "modem"), have you considered a PoE adapter to get power from where your main network setup is?

author: Sercan
author_rank: Contributor
timestamp: September 21, 2025, 03:27:29 am
content: @NiHaoMike thanks for the suggestion. I already have mains power at that location, so powering the converter isn’t a problem.