DIY very portable battery pack - will it work?

Samyham

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I'm not sure it's doable but I would like to follow through on making this battery pack. I have a General license but don't have much actual experience. I just bought an FT-891 a couple of months ago but still haven't used it.

I got this idea from watching Radio Prepper on utube. I bought 2 battery holders on Amazon.com. That's a total of 16 AA batteries. At 2400 mah (read 2.4 amps/hr) per battery we have a total of 16 X 2.4 amps = 38.4 amps. X 12 volts = 460.8 watts per 1 hour total capacity.

In the unlikely but possible use of Yaesu FT-891 radio at full 100 watts non-stop uninterrupted, those batteries, in theory, should last 4.61 hours. In reality, because of inefficiencies I swag it at about 70 -80% of that.

First, is my math correct? If not, please correct me. Second, the wires individually I am guessing are a little smaller than 18 ga and soldering the 2 holders terminals together in parallel would make them resulting in 2 leads of just over 18 ga. each. Will these lead wires withstand such a draw? It seems to me that the wires, just over 18 ga., would be too thin and would over-heat up and melt - or something worse like damage to the radio.

Please review and explain. Thanks very much.
 

ramal121

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Your math is a little off. 2400mAh, are we talking alkaline batteries here? Well first, one battery holder is 8 batteries in series to give a total of 12 volts but does not increase the capacity which would still be 2.4 Ah. 2 battery packs in parallel would boost you to 4.8 Ah total. not 38.4 Ah, or 57.6Wh.

I'd say a 100 watt radio will pull 20 amps on transmit. Given this your batteries would last maybe 15 minutes.

The kick in the pants is the internal resistance of alkaline batteries causes a huge voltage sag, probably under 1 volt per cell, whenever you try to pull more than 1 or 2 amps out of them for even the shortest amount of time. Study up on the Peukert effect.

The 18 gauge wire has no problem of overheating because I think the batteries would not have enough oomph to make them even slightly warm.

100 watts on battery, get one with some serious Wh capacity.
 
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mmckenna

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Yeah, AA batteries isn't going to give you enough. If you cranked the power way down, to 5 watts, you might get this to work.

12 volt Gel-cell would be a better option for cheap power.
If you want to get fancy, there are some expensive options with higher power density, but that doesn't sound like what you are looking for.
 

K4EET

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Hi @Samyham, let's see what we have here. To expand on what @ramal121 said, the Yaesu FT-891 manual lists the following specifications:

Supply Voltage is:
13.8 VDC +/- 15%

Current Consumption is:
Receive with signal present is 2 Amps
Transmit at full power is 23 Amps

I agree with @ramal121 that the batteries themselves will not be able to handle the current draw. While the transceiver will probably not draw quite 23 amps on transmit, still an alkaline "AA" battery bank is liable to explode. The internal battery resistance is going to generate heat and release a vapor that is suppose to escape through vent holes. However, due to the amount of current that you are talking about, that heat may build so fast and the vapor not vent fast enough... Well, let's just say that given the right set of conditions, you will have an explosion and fire on your hands. That you do not want.

You really want to be using deep discharge cycle gel cells. Here on Radio Reference there are threads that discuss what you need to be thinking about. On the Internet there is a boatload of information as well. Here are some threads:





Take a look at those threads and see if you can get some ideas. Let us know if you have any questions.
 

tweiss3

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Bioennio put together some really good charts for usage time, both for normal usage and for "field day" usage:
2080 Standard Duty Chart
5050 Field Day Duty Chart

I recommend looking through their FAQ as well, as there is good information on batteries and systems: FAQ

You aren't going to get where you want to be using AA batteries. It is possible to build your own battery pack, but you won't save much money, if any at all. Look at the pricing and capacity of the Bioennio LiFePO4 batteries and compare to building your own, you'd be supprised.
 

sloop

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I understand the desire to "DIY", but sometimes the simplest (and cheapest) way to get something is the most direct way. Just buy a 12 volt lawnmower battery, it will provide all the voltage and amps that you need for your radio and a cheap battery charger will keep it charged. If you shop right it will probably be more cost effective in the long run.
 

Samyham

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Thank you all very much for your thoughtful replies. I will answer somewhat in the order of the replies.

Ramal121, I am embarrassed to admit I forgot to consider the battery holders were in series! Doh! Although I bought the holders which are cheap, fortunately I followed my hunch and held off buying 16 double A batteries.

mmckenna, I'll have to look more into the batteries you recommended. But you are right if the batteries aren't small and inexpensive. I will have to look into the Gel-cells you speak of.

K4EET, hey old timer! I have not yet looked at your links. I wanted to review these replies and respond to them first. But thank you very much for the links. I know they will be very useful.

tweiss3, this is the first I have heard of Bioennio. They look to be top quality. I'll keep them in mind for future. A tad expensive but compared to what I spent buying the FT-891 it is small.

Sloop, you're right. I already have an Interstate DCM00035 35Ah deep-cycle agm wheelchair battery. Great minds think a like! However, it's not quite backpack portable. But I think mulepack portable plus some carrots is what I will have to settle for.

I am left asking a question. My Yaesu FT-891 max's at 100 watts. Using the straight-forward formula for watts, we divide by 12 volts to get 8.33 amps. But as Ramal121 and K4EET point out it's actually multiples of that. What am I not seeing?

Thanks very much again to all,

Samyham
 

tweiss3

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Well, remember watts = volts * amps, and tranceivers work on 12v (to keep things simple) It's easiest to keep things in watts first.

Standard usage assumes 20% transmit (100 watts) time and 80% receive (5 watts) time, which averages to 24 watts (0.2*100 + 0.8*5). That gives the following from the Bioennio chart:
1614209899547.png
Using your 2400mAh (2.4Ah) battery packs in parallel, you need 10 full packs to last an hour.

Or, you could buy a basic 12Ah battery for $125 bucks 12V, 12Ah LFP Battery (PVC, BLF-1212AB)
Keep in mind, you would need 50 of your packs (8*50=400 AA batteries), which very quickly would cost significantly more than the Bioennio battery.
 

mmckenna

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I am left asking a question. My Yaesu FT-891 max's at 100 watts. Using the straight-forward formula for watts, we divide by 12 volts to get 8.33 amps. But as Ramal121 and K4EET point out it's actually multiples of that. What am I not seeing?
100 watts RF output ≠ 100 watts DC input. They are not 100% efficient.

for a 100 watt radio, you are probably looking at something around 25 amps of current draw while transmitting.
 

AK9R

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Every radio I've seen from Alinco, Elecraft, Flex, Icom, Kenwood, or Yaesu has a published set of specifications which includes the voltage requirement, usually with a tolerance, and the maximum current while transmitting. Some of these specs also include the current draw while idle or while receiving.

There's no need to guess or make assumptions about how much power your radio needs. Look at the specs.
 

K4EET

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Every radio I've seen from Alinco, Elecraft, Flex, Icom, Kenwood, or Yaesu has a published set of specifications which includes the voltage requirement, usually with a tolerance, and the maximum current while transmitting. Some of these specs also include the current draw while idle or while receiving.

There's no need to guess or make assumptions about how much power your radio needs. Look at the specs.
I posted that for the OP's transceiver in Post #4 above... 73, Dave K4EET
 

Samyham

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Thanks to all again!

Yes you did, K4EET. 23 amps it is, in the specs. From the battery's 'point of view' I'll need to consider this in my calculations. But I also like the idea of 20/80 average split between Rx/Tx. This has been very educational for me from a practical point of view. Once I get to using my wheelchair battery I'm sure it'll be even better.

73,

Samyham
 

K4EET

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<snip> Once I get to using my wheelchair battery I'm sure it'll be even better. <snip>
Are you in a chair? If so, I am too. I have a Pride Mobility / Quantum Rehab Q6 Edge right now. It is on its last leg so I'll be getting a new chair soon. What I wanted to say, If you have a 24 volt system, do not tie a 12 volt transceiver to one half of the batteries. That will unbalance them. You will want to use a 24 volt to 12 volt DC to DC converter. 73, Dave K4EET
 
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