# Is this battery of gel or AGM type?

Here is a picture of the battery.

I intend to use this battery as a backup power source for my PTCL DSL modem. The modem which I possess is rated for 12V 1A DC. The actual consumption which I have measured using a digital multimeter is around 230mA. I'm trying to calculate the minimum capacity of a battery which would power the modem for 8 hours of load shedding.

Capacity needed for powering up the modem for one hour = 230 mAh
Capacity needed for powering up the modem for 8 hours = 230 mAh * 8 = 1.84 Ah
Since a lead acid battery should not discharged more than 80%, thus capacity needed = 1.84 / 0.8 Ah = 2.3 Ah

Is this 2.3 Ah the required minimum capacity?

Assuming the power goes out for 8 hours consecutively, will a fully charged 2.3 Ah (rated @ 20hr discharge) SLA battery power the modem for 8 hours?

At 80% depth of discharge assuming continuous cycling, a 2.3Ah battery would have a very short life. If you use PTCL EVO USB device with it (or anything plugged into USB port), current consumption will increase.

As such, assume consumption to be 0.3Ah (consumption will vary based on battery output voltage). For 8 hours, it will be 2.4Ah (8 x 0.3Ah). Use estimate of 50% depth of discharge (for good battery life) so you will need minimum 4.8Ah (2.4Ah x 2) capacity. Get any battery having capacity above that (5.5Ah+). The pictured 7.5Ah battery will work just fine. GEL/AGM type selection is not critical. Either will do.

You will also need a suitable charging mechanism and a low voltage cutoff mechanism to ensure battery is not damaged due to overcharging or excessive discharging. Also use an inline fuse (1-1.5A rating) and diode (1N400x series standard or any low leakage schottky) to ensure that mistaken reverse connection does not cause your modem to give out the magic smoke.

^ Same as above.. or just buy a small 30AH local wet battery.. charging it wont be an isuue, as hazaar type desi and imported chargers available for it, it will be cheaper than a comparable Dry battery, as well as a LOT more backup. it will also sell well compared to that super small dry battery.

I will be making a 3 stage charger with undervoltage protection for this battery.

[quote=“mysterio92, post:4, topic:22203”]

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AGM.

its an AGM Battery ........ Gel batteries dont come in small ratings ........... all 7 ampere batteries are AGM .........

[quote=“APC_UPS, post:6, topic:22203”]

its an AGM Battery … Gel batteries dont come in small ratings … all 7 ampere batteries are AGM …

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What are some other ways using which one can recognize a gel type battery?

well bhai its a simple scenario .... whenever u come across any dry battery .. there ll be written its manufacturer and model ......... just type it in google n it ll surely show u its type ......

Oh, I thought there might be some other way other than looking over internet like by just looking at battery exterior.

no yaar .... all VRLA battery whether its agm or gel ...... all look same ....... apart from their design and footprint

PTCL modem's power adapter has built in voltage regulator in it which delivers a steady 12.0 volts to modem. Your battery powered set-up may deliver from 15 volts (battery max charging voltage) to less than 10.5 volts (at battery fully drained). You should put a voltage regulator circuit (e.g. LM2940 12 volt 1 amp) between the battery and the modem. See info available at: http://www.reuk.co.uk/LM2940-12V-1A-Low-Dropout-Regulator.htm. Parts are available locally. This will ensure that modem always gets between 11.5 - 12 volts whether battery is charging or not, providing battery is at least at 12.0 volts. When battery dips below 12.0 volts, the circuit will cut-out and save the modem. See following link for useful battery calculation info: http://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html A 12 volt 7.5 AH battery will NOT be OK as the battery voltage will drop below 12.0 volts after only a few hours. You need a bigger battery or your PTCL modem should be able to operate on low voltage (10.5 volts is the defined cut-off point for C20 Amp-Hr test), which in your case would be 10.0 volts output after LM2940).

I took some reading. Here are the results:

Do you want modem to run on raw battery power (from less than 10 volts to more than 15 volts depending on battery's state of charge and your charging system's output) and risk reduced life / quick failure of modem? If yes, go ahead and try your luck.

If no, operate it within same voltage range as it gets from adapter supplied with it (11.25-13.83 volts) by never charging battery when connected to modem and disconnecting battery when it gets to 11.25 volts.

Here is my UPS.

It has got battery undervoltage protection, charge current limitation, and zero transfer time.

With the same fully charged battery shown in the first post, the modem ran for 10+ hours with cut off voltage set at 12 V.

Anybody willing to make one for himself can find more details on my blog.

Make Mini UPS For DSL Modem PART-I - Introduction

Make Mini UPS For DSL Modem PART-V - The Circuit Diagram