APC UPS 550 question regarding battery

EquineHero · May 14, 2016

I have an APC Back-UPS 550 with a dead battery. I'm not a genius, but as far as I'm aware if you're creative enough laptop battery cells can power anything. I've gotten a radio, a 14V external computer monitor, a router, and a whole bunch of other things to run off of a varying amount of laptop lithium ion batteries. I understand that doing so is dangerous and do so knowing the risks of fire, explosion, et cetera.. I also know that the battery inside my APC 550 is 12 volts. The laptop cells I have are 1.5 volts, 8 of them should be sufficient to replace the lead-acid battery - but I want to know the exact risks and potential disasters of putting so many batteries in series. I have an old Dell that has a bad battery that I extended with 6 additional cells in parallel and as far as I can tell it's doing fine. This uninterruptable power supply is actually really important, so if someone can help with this that'd be great. I also know that I can buy a replacement for $10 on Amazon...but I don't care.

InvalidError · May 14, 2016

You cannot connect lithium batteries to a lead-based battery chargers because the charging requirements are different: lead batteries can tolerate a temporary gross over-charge and continuous trickle-charge while most lithium batteries will fail potentially catastrophically if over-charged. Similarly, lead batteries can recover from over-discharge while over-discharge may immediately ruin lithium-based batteries.

In other words, you cannot put lithium cells in a device designed around lead batteries. At the very least, lithium batteries require over-charge/discharge cut-off circuitry that isn't critical for NiCd, NiMH and lead batteries and even if the device already has that circuitry built-in, you still need to adjust the cut-offs to what your specific battery needs.

You also need a charge balance circuit to prevent the state of charge from going too far imbalanced in the battery pack or you may end up with a cell failing catastrophically without warning. With NiCd, NiMH and lead, pack imbalance can be sorted out with a trickle charge which is not possible on lithium.

molletts · May 14, 2016

Don't even attempt it. Messing with lithium-ion cells is extremely dangerous unless you know exactly what you're doing. If the UPS is really important, you don't want it to suddenly blow up and burn your house down, which is the likely outcome with lithium cells. Come to think of it, you probably wouldn't want it to do that even if it wasn't important.

Replace the lead-acid battery in the UPS with another similar one. You can pick them up cheaply at many places including some DIY-type stores (they're often used as backup batteries in burglar alarms and the like). Yuasa NP-series batteries work very well in UPSes in my experience but their cheaper Yucel range are also quite good.

EquineHero · May 14, 2016

InvalidError :

So what if I built a circuit (or purchased one somewhere) to accommodate for overcharge? Many laptop cells are pretty much the same, being 1.5 to older ones being 1.7 per cell. Are there schematics for an adjustable overflow cutoff?There's plenty of room inside the UPS casing for additional circuitry, and IIRC lithium polymer/ion batteries have a longer overall charge/use cycle.

InvalidError · May 15, 2016

Rechargeable lithium cells should be 3.0-3.7V per cell while under load. If it drops to 2V, the cell is well beyond dead.

Yes, there are premade load balancers out there that you may be able to stuff in to take care of that. For the over-charge/discharge protection, finding a premade one designed to handle 50A and has the correct cutoffs for the cells you plan to use may require a lot of digging around.

To supply 50A, you will also need a handful of reasonably well matched cells in parallel if you are going with 18650s.

EquineHero · May 15, 2016

InvalidError :

Even if the cell has "1.5V - do not disassemble" written on it, should it still be 3 to 3.7? AFAIK 3.7V was the standard for cellphones for a very long time until Qualcomm decided to make rediculouse quad core battery eating processors. These are the individual cells found inside laptop batteries. These things:
http://sell.pakuya.com/upload/20120104/Supply_Replacement_Repair_Laptop_Battery_Cell_2200mAh.jpg

Since the lead acid batter [strike]is [/strike] was 12 volts and 7 watt-hours, 8 1.5 cells should work. I should probably account for amperage somehow, any ideas?

InvalidError · May 15, 2016

EquineHero :

Rechargeable lithium battery voltage is a function of battery chemistry. It was around 3.7V 20 years ago and is still around 3.7V for most lithium battery variants in use today. If you read the print on the cells themselves, it says 3.7V there too. If a rechargeable lithium cell gets discharged to 1.5V, it is usually damaged beyond recovery and only good for the scrap/recycling heap.

EquineHero :

It isn't watt-hours, it is watt-hours. 12V x 7Ah = 84Wh. If you use only two strings of batteries in parallel, the UPS may draw over 25A from each string assuming they are perfectly matched. The cells may not like that, you need to find their spec sheets to find out what their safe maximum continuous discharge rate is. In many cases, this will be well under 10A and you would need at least five strings of batteries in parallel.

EquineHero · May 15, 2016

InvalidError :

I have 15 available cells, and I'll have more third week of July. My ammeter confirmed that it is in fact 3.7 volts, my bad. One of the reason why I'm not going to buy a replacement is because I have almost no money, I work a near dead end job and my garage is full of old electronic stuff I've collected from scrapyards and recycling centers, which I plan on doing things with instead of ending up on Hoarders. I'll be testing each cell to make sure it charges to at least 3.5. How many should I put in each parallel string, and how many strings should I have? I am potentially going to acquire at least 45 more cells, and summer is almost here, which means power outages. I mostly just use my laptop and 90W charger.

InvalidError · May 15, 2016

You don't have money to buy a proper $25-30 replacement but you can afford 50+ dead/semi-dead unknown condition used cells, over-charge/discharge circuitry, a lithium cell charger with charge balance circuit?

You cannot put a random assortment of cells in different condition, origin, quality, slightly different chemistry, slightly different capacity, etc. in a pack, you'll end up with a few cells over-charging/discharging and potentially failing catastrophically. You should research proper lithium battery safety, charging and discharging before attempting to go any further with this.

Safely using lithium batteries require more thought than just putting a bunch of cells together in series-parallel configuration.

EquineHero · May 15, 2016

InvalidError :

I didn't buy these cells, and I know for a fact that they are all good. A friend of mine in GB is sending me a lot of 20 used laptops to tinker with. I plan on building the needed circuitry myself from schematics online. Everything else is irrelevant, I just wanted to know the dangers,

InvalidError · May 16, 2016

Being "good" is not good enough, they have to be a similar level of "good" to ensure that charge balance and charge/discharge current is reasonably even across the whole pack. As I wrote earlier, it takes only one bad/weak cell to ruin your day/life.

Ideally, you shouldn't even mix cells that come from different production batches.

molletts · May 16, 2016

EquineHero :

[Oops, didn't mean to upvote the message higher up in the thread. Slip of the finger while scrolling

]

You say they're all good but then you say they're used. That doesn't make sense.

I assume you're an experienced electronic engineer, used to working with high currents and potentially explosive lithium cells.

You'll need to balance the voltage and charge current for all the cells, preferably individually if they're not all identical and brand-new, and ensure that they do not exceed their rated discharge current. You should also monitor cell temperatures - they will likely be running very hot in a high-current application like a UPS. (I've had even lead-acid batteries boil and explode in UPSes before but it's only a bit of sulphuric acid and a nasty smell you have to deal with when that happens.)

You will probably spend more buying suitable high-current MOSFETs, chokes and suchlike than you would buying a new SLA battery or two for the UPS, not to mention the time you'll spend on it (and possibly the impact a few fires will have on your insurance premiums).

EquineHero · May 16, 2016

molletts :

Damn my mouse to the 7th Plane of Torment, stop clicking the back button when I finish a 3 paragraph reply!

I have over 9 years experience working with 120+ volt currents, I'm just being cautious. I once powered a Razor electric scooter with a car battery. The results were...amusing.

I have all of the components required to create an overflow cutoff and a choke/variable resistor.

I will test all of the cells individually when the arrive, anything less than 3.5 volts at full charge is out. The laptops are all no less than 1 year old and no more than 2, and most of the batteries are actually OEM replacements or compatible extensions.

The reason why I want lithium instead of lead acid is because lithium cells have a higher charge recycle tolerance than lead acid (they last longer, at least in my experience). If that was not the case we would have lead acid batteries in our phones.

InvalidError · May 16, 2016

EquineHero :

Wrong. The main reason we have those more dangerous lithium batteries in portable and mobile electronics is because lithium batteries weigh less than half as much and are less than half the size of a similar capacity lead-based battery, which makes lead a non-contender for any weight or space-constrained application. Lithium also has the benefit of being much more environmentally friendly than lead.

EquineHero · May 16, 2016

InvalidError :

EquineHero :

Wrong. The main reason we have those more dangerous lithium batteries in portable and mobile electronics is because lithium batteries weigh less than half as much and are less than half the size of a similar capacity lead-based battery, which makes lead a non-contender for any weight or space-constrained application. Lithium also has the benefit of being much more environmentally friendly than lead.

Oh, okay. Can you answer the rest of my questions, then?

InvalidError · May 16, 2016

EquineHero :

You said you planned to build the circuit yourself. You have been made aware of the possible risks of failing to do so correctly, now you need to do the necessary homework to come up with a safe design.

How many cells in parallel? That depends on the cell's maximum discharge rating. How many cells in series? Three or four but you will need to hack the UPS' charging circuitry either way. The rest depends entirely on what other random bits and pieces you get your hands on and you are on your own there.

EquineHero · May 16, 2016

InvalidError :

EquineHero :

You said you planned to build the circuit yourself. You have been made aware of the possible risks of failing to do so correctly, now you need to do the necessary homework to come up with a safe design.

How many cells in parallel? That depends on the cell's maximum discharge rating. How many cells in series? Three or four but you will need to hack the UPS' charging circuitry either way. The rest depends entirely on what other random bits and pieces you get your hands on and you are on your own there.

Thank you so much for all the help.

Lunas · Aug 26, 2016

EquineHero :

Li-ion are supposed to be 3.7 to 4.2 fully charged about 2.8-3v is dead if they are below that they are damaged and should be disposed of.
In theory you should be able to make or get a charge circuit for it that will adapt it but issues arise with discharges and honestly you really don't want to have to rely on this as your solution you would be better off using a small car or motor cycle battery. Deep cycle batteries might be the better option and if you want to know how solar is stored a bank of deep cycle batteries wired in 12v 24v or 48v configurations connected to an inverter to take one of those voltages and output 120v Ac @60 hz

APC UPS 550 question regarding battery

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