Re: 2Ahr

Rick Bennett

Michael, another thought is to get a 10 cell AA battery holder and use 10 AA NiMH batteries. This will be somewhat lighter and smaller than the SLAB. Also the NiMH batteries will hold their voltage to a pretty deep discharge and the NiMH batteries don't mind being run low over and over. (One of the disadvantages of SLABs is they don't tolerate a lot of deep discharge cycles). And in a pinch, you can substitute alkaline batteries. I used a similar setup years ago for portable ops with good success.

To go along with Greg's suggestion below, if you can put an ammeter on the power input and an rf power meter on the output at the same time you can look for a good balance of power in to power out. Even 3 watts is a respectable power level if you can get a good increase in battery time.

de KC0PET, Rick

----- Original Message -----
From: "Greg Troxel gdt@... [4sqrp]" <4sqrp@...>
To: "Michael McEwen" <mcewenk5osa@...>
Cc: 4sqrp@...
Sent: Tuesday, June 16, 2015 5:25:39 PM
Subject: Re: [4sqrp] 2Ahr

"Michael McEwen mcewenk5osa@... [4sqrp]" <4sqrp@...>

I've found a neat little 2AHr SLA (Off road ATV) and it is easily in my PM
weight range. I have an LD5 CW/SSB TXCVR. Would this little batt allow a
couple hours of typical SOTA/PM ops? As I read the LD5 "manual", it should
due to 2A draw on TX...but, I'm an electro-dummy and may be wrong.
A rating of 2 Ah means that it will supply 2Ah/20h = 100 mA for 20h
before reaching 10.5V. As the discharge rate increases, the available
Ah decreases and vice versa, and you get greater voltage drops due to
internal impedance. Generally published curves are available for
multiple charge rates. I would guess that if you were drawing 2A
intermittently with 0 the rest of the time, it might go 30 minutes.
Assuming 0.3A for receive, and 50% keydown while transmitting, the
notion of transmitting for 20 minutes and a few hours of receiving does
not sound crazy.

Hoewver, there's no substitute for actually measuring the current while
in continuous keydown, and in receive, and using an integrating ammeter
to measure total Ah used over some period of operating. I ran a 2m FM
net (at 15W output) as NCS for about 15 minutes total operating time,
and remembered using only about 1 Ah. But this was from a 35 Ah
battery, so my currents were closer to C/20h than our example (0.5A rx,
5A tx, more or less). I'm not sure I would have made it on a 2Ah
battery, and I wouldn't have tried.

The other thing is just to try it; the worst is that you'll break it (if
you discharge too deeply) or that you'll have to stop operating.
Definitely keep an eye on the voltage and just stop if you see anything
below 11.4V on receive.

These days, you are arguably better off with a LiFeP04 battery of about
the same weight and higher capacity, and better ability to cope with
high currents. But it will cost more.

73 de n1dam

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