Other SOTA sites: SOTAwatch | SOTA Home | Database | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

S.L.A.B. Question


#1

Hi All

Since my 2m FM activations have been “interesting” here down South, I will try to use the 706 next time out for 2m SSB and/or HF. With that in mind I’m looking to get a 7AH SLAB as that would seem the ideal for what I need/intend.

However, I’m a little confused over the charger required and the best protocol to keep it “well charged” but not “overdone”.

Does anyone have some experience they would like to share on either issue?

Thanks and 73 Marc G0AZS


#2

In reply to G0AZS:

I’m in the process of writing an article on just this for Radcom. In essence you need a contant current charger which you can make with an LM317T and a resistor - junk box components. Set the open circuit voltage to 14.5 Volts and the charge current to 1/10C or less and you will not damage the battery. Once the terminal current reaches 14.5 Volts or very close, you can simply connect the battery across your standard shack power supply, perhaps with a small series resistor, to keep it float charged at 13.8V.

Less sophisticated is to simply put a resistor in series with your standard shack 12V supply to limit the inrush current to the battery and float charge that way. That gives a slower charge time but is also safe.

As with all high current devices - proper fusing is essential!

73

Richard


#3

and the charge current to 1/10C or less

Thanks Richard.

Sorry for the additional question but what did you mean by the above? i.e. what is 1/10C?

73 Marc G0AZS


#4

In reply to G0AZS:

1/10th of the capacity of the battery.

See:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page7.htm

LM317T charger halfway down page.

73

Richard


#5

Thanks again…

…of all the reflectors I subscribe to related to Radio, this has to be the most useful, friendly and courteous!

73 Marc G0AZS


#6

In reply to G0AZS:

Marc if you intend using your 706 on high power (not sure if it`s 100w or not)then I would advise using a “deep discharge” type slab. The normal Yuasa type are not suitable for providing heavy current as they are designed for standby use in intruder alarms. I got mine from rapid electronics (no connection)

http://www.rapidonline.com/productinfo.aspx?tier1=Electrical+%26+Power&tier2=Batteries&tier3=Lead+Acid&tier4=Deep+discharge+gel+batteries&moduleno=61882

If you`re not too practical & financially solvent, they also sell purpose made intelligent chargers.


#7

Hi Steve…

I’ll bear that in mind and thanks for the link.

I had thought that I would try and use 10W usually and maybe 20-30W maximum if condx really needed it.

73 Marc G0AZS


#8

In reply to G1INK:

Steve

The data sheet for the Yuasa NP7-6 states a maximum discharge current of 35 Amps - surely that’s enough for even an IC706!

73

Richard


#9

In reply to G0AZS:

You charge lead acid batteries with constant voltage. This is different to NiCd/NiMH cells which use constant current. So to charge a lead acid cell you can use a stablised powersupply. i.e. you could charge up a 12V SLAB from a 13.8V supply you probably already have.

But… 13.8V is not enough to full charge the SLAB., probably you will only get to 75-80% charge with that voltage. The actual charging voltage depends on the use for the cell, a lower voltage is used if you intend to leave it on charge forever and occasionally use the cell. For your use, you can charge the cell with a higher voltage and disconnect when fully charged. In this case 14.3V will do nicely.

The initial current when placing a 7AHr cell on charge will peak high, possibly in excess of 10A or more, but this will rapidly drop. The charge current drops as the cell charges. I use a 2.8AHr cell and after an activation the other day last over 1hr using SSB on HF and a good 30mins of 2m FM nattering, my cell pulled over 5A for 30secs until the current dropped to about 2.1A. It took a couple of hours to slowly drop to about 50mA at which point I removed it from charge.

If your PSU cannot supply the charge current then you will put the cell into slope charge. If your PSU has a current limit, it will lower the voltage to keep the current in range. In this case as the cell charges, the current drops and so the PSU voltage will rise. This is not good for maximum life of the cell. However, if the cell is only in slope charge for a few minutes, it’s not worth worrying about. For a 7AHr cell, you probably want a supply capable of providing at least 3A without limiting.

The other thing to keep an eye on is the temperature when charging. The higher the temperature, the lower the charge voltage. Probably not a problem as long as the temperature is less than 20C. But once it gets above that you need to lower the volatge. The manufacturer’s data shows the derating curve. Most lead acid cells are killed off prematurely by charging at too high a volatge for the temperature. It is what kills off car batteries in hot countries more than anything. Charging at the voltage specified for 0C when the temperature is 30C can reduce the number of charge cycles the battery will sustain by a factor of 10 or more!

Finally, don’t leave the cell discharged. Always charge it up fully as soon as you can. Otherwise the plates will sulphate and you will find you cannot charge the cell or the capacity drops to a fraction of its original capacity. You should charge up the cell every 6 months if you are not using it as it will slowly discharge and start to sulphate.

Andy
MM0FMF


#10

In reply to MM0FMF:

Sounds like we agree!

73

Richard


#11

Thanks Andy…

…if I dont want to do any construction it sounds like my spare, unused 13.8V 25A power supply in the shack would happily charge it to the 75-80% capacity you mention.

However, if I keep doing this (i.e. charge at less than 14.3V all the time), is there any long term effect on maximum capacity?

73 Marc G0AZS


#12

In reply to G0AZS:

Well the only real downside is that if you never achieve full charge state then you will slowly allow the plates to sulphate. This has the effect of reducing overall capacity and ability to charge. However, it will be a slow process compared to leaving the cell discharged for long periods.

With the relative low cost of the cells, it’s not a big financial burden if you only get 375 charge cycles out of a £10 battery compared to 750 if you fully charge it. However, if you occasionally are able to fully charge it, you can increase the life.

There’s nothing to stop you opening up your PSU and tweaking up the set volts control to lift the voltage to 14.3V to fully charge the cell now and then. You just need to remember to set it back to 13.8V. Though to be honest, most equipment designed for 13.8V works happily upto about 15V. Certainly, the voltage in a car when the engine is running will be 14.4V and you don’t see equipment bursting into flames because the power rail is 0.6V higher than a PSU in the shack. In fact, you’ll probably find the average ham rig is more linear at the higher voltage!

Andy
MM0FMF


#13

This link is useful:

http://www.batteryuniversity.com/partone-13.htm

73

Richard


#14

In reply to G0AZS:

Marc I have emailed you the Yusa slab info .pdf file, which has all the answers you require re: slabs.
I use Yusa NP7-12 7ah slabs and charge them with a Powertech automatic charger, it’s the plug in the wall type and runs at 350mA 4.2VA.
It has a red led on the front which illuminates solid at first after initial switch on and the battery draws most current, then it starts flashing and goes into trickle charge mode as the battery’s voltage rises.
The charger was only about a tenner on ebay so not worth constructing one. Model number is BC-12500, google it for info.
Prior to my activation of GW/NW-070 yesterday, my Yusa slab read 14.4v fully charged and after the activation, approx 1 hour with FT-817 and 5 watts, the slab was still reading at 13.3v this morning.
Bear in mind the FT-817 only draws 450mA on rx and 2 amps on tx @ 5 watts.
Your 706 will probably draw somewhere in the region of 2 amps in rx at a guess.
The bottom line is, yes the 7ah slab is fine for your activations with the 706.

73 Mike GW0DSP


#15

Thanks Mike… very useful… I have the PDF now.

…and yes the 706 does draw about 2A on rx… gets a little warm just monitoring! Goodness knows why it is so inefficient.

I wont rely on it in the longer term though. My plan is to build a K1.

73 Marc G0AZS


#16

In reply to G0AZS:

I wont rely on it in the longer term though. My plan is to build a K1.

Now your’e talking!!

Mike


#17

In reply to G3CWI:

In reply to G1INK:

Steve

The data sheet for the Yuasa NP7-6 states a maximum discharge current
of 35 Amps - surely that’s enough for even an IC706!

73

Richard

That may be so Richard, however if you persist in drawing heavy current during activations & recharging, you will soon end up with plates more bent than a nine bob note. Much better to utilise the deep discharge type (with solid plates)that are designed for this use.

72 Steve G1INK/QRP.


#18

In reply to GW0DSP:

Hi Mike
Where did you buy you Powertech Auto charger from as need one myself?

I also have the ICOM-706-M2G and I run that off a car battery when on the local summit TW-004 but I only run it at 5-10watt and tends to work well but not a rig I would use for putting in the rucksack.

de G0VWP


#19

In reply to G0AZS:

Yes you are right Marc the 706 does get warm on RX I have one and its in the shack and also on all day from around 0700hrs-2300hrs I have never had any bother with it at all I got it in 2004 and in constent use evry day.

de G0VWP


#20

In reply to G0VWP:

In reply to GW0DSP:

Hi Mike
Where did you buy you Powertech Auto charger from as need one myself?

Hi Terry

I bought it off ebay years ago, originally to charge a slab to power a glowstick and engine starter for model aircraft flying.
I’ll see if I can find any info of the shop that sells them and email it to you.

73 Mike