120hr backup for 800w

[bk81]

Hi everyone,

Don't know if I'm in the right section for this. I've been asked to find out how many battery strings it would take to provide 120hrs of backup for an 800w (16.67A @ 48vdc) load.

I have 4 x 12v 100Ah batteries connected in series and then each string in parallel. I need around 10% of the battery current rating to allow for recharge and also The battery voltage can't go below 42vdc before the low voltage disconnect kicks in.

Any ideas? If anyone knows a formula that would be even better :wink5: cheers.

 

[liquidity]

What you're saying is that you want to draw 16.67A for 120 hours. For sizing purposes that's 2000A for 1 hour. So you'll need 20 sets of those 4x12v 100 Ah batteries plus further batteries to compensate for losses and the inevitable degradation that occurs over time. That's a lot of batteries.

 

[TedM]

Manufacturers' recommendations are usually to never go above 2 or 3 strings and only discharge to a max 50% (both in order to prolong life) so you need to select batteries with an appropriate Ah capacity accordingly.

800 W x 120 hours = 96000 Wh
96000 / 48 V = 2000 Ah
allow for max 50% discharge = 4000 Ah
4000 x 1.2 temp co-efficient (assumes 10C) = 4800 Ah
4800 / 3 strings = 1600 Ah per string

So you need to select a set of batteries with a 1600 Ah rating.

I'm assuming they are going to be kept in an unheated building. If they are kept around 20C then you can drop the temp co-efficient to 1.0 or if they may be kept at temps as low as 0C then up it to 1.4.

 

[grantr37]

Hi everyone,

Don't know if I'm in the right section for this. I've been asked to find out how many battery strings it would take to provide 120hrs of backup for an 800w (16.67A @ 48vdc) load.

I have 4 x 12v 100Ah batteries connected in series and then each string in parallel. I need around 10% of the battery current rating to allow for recharge and also The battery voltage can't go below 42vdc before the low voltage disconnect kicks in.

Any ideas? If anyone knows a formula that would be even better :wink5: cheers.

what's this backup for if that's ok?

 

[ExtremePV]

Can I ask what the system is for?
Yes you'll need alot more than the 4 x 12v batteries you have. For this size of battery bank approx 3000Ah @ 48V you'll need to use 2V cells - you don't want to put more than 3 battery strings in parallel.

The other question is how are you going to charge these batteries?

Ken

 

[bk81]

For some reason I didn't have any notifications of these replies, sorry for the info blackout!

The customer went wildly over on his assumptions, it changed to 200W so we're now offering 4 strings of 170Ah, as it's got to work
at 0°C.

We can only take the the cells to 1.85Vpc because of the duration of discharge, hence the 4 strings, a lot of going back and forth to the manufacturer to back all this up, good learning experience though!

Here's the calcs I used;

200W/(24 cells x 1.85Vpc) = 4.504A

4.504 x backup time (120 hrs) = 540.5Ah

540.5 x temperature correction for 0°C for 120hr discharge (0.88) = 614Ah

614/Ah of battery (170) = 3.6 so I'll need 4 strings of 170Ah to cover 200w for 120hrs at 0°C.

The system is to cover emergency services radio signal, that's the reason they need such a long backup time with little load.

Cheers for the responses!

 

[TedM]

You've left out the step where you account for only discharging the batteries to a 50% level so you need to at least double what you have calculated.

I would suggest you need 3 strings of 400Ah batteries.

 

[bk81]

Hi Ted,

If you look at the data sheet for an Enersys 12V170F battery it says you can discharge 8.4A for 24 hours per string so roughly 1.68A for 120 hours by taking the battery to 1.85Vpc, according to Enersys 4 strings (680Ah should handle it).

http://www.enersys-emea.com/reserve/pdf/EN-VFT-PD-010_1012.pdf

I admit I may not be right as this is by far the longest autonomy I've had to do, but if the battery manufacturer is willing to guarantee it, and the life time of the battery down to 0°C, then they better be right considering what it will power!

Thanks for the help on this, cheers!

 

[bk81]

Enersys just got back to me, so it seems I'll be discharging the battery to 20% if I use 170Ah or 29% if I use 190Ah, so I think to be on the safe side I'll use the 190Ah.

It will, I think be extremly rare for the battery to be discharged for 120 hours so if they need to replace some of them after a 5 day blackout then so be it! I would think whoever is maintaining them would carry out extensive battery checks after such a discharge anyway??