Battery help please

[VK2AXL]

I'm hunting advice folks. I am going rucksack portable with an Elecraft K2. I was thinking of getting a gelcel battery for power. But I'm not really up to speed on recent battery technology.

A while back I asked a similar question about my FT817, and I was advised to use LiPo's. This is a slightly different scenario. Space and weight is not such an issue. I wont be walking very far and I will easily have space for, say, a 7AH gelcel. However, cost is an issue.

Battery and charger requirements are- inexpensive - easily sourced - simple.

What is the best way to charge a gelcel? I know this is a contentious issue and will likely result in multiple theories being put forward. But the keyword here is... inexpensive. I would like one of those digital smart chargers, but this project does not stretch to 100s of $$s.

The rig is about 1.5A on TX, 0.2 on RX, and I need to keep it on air for 3 to 4 hours. Does anyone have any good ideas?

[ZL2WHO]

Hello Jack.
To keep it simple and the cost low I would use an LM317 reg.
Main criteria is not to over charge the battery and to limit the charging current so the reg doesnt burn up. If you need to maintain 200mA on Rx then id be thinking to charge the Batt at say 500mA. Iv always used the 10% rule for charging, ie a 7AH batt should not exceed 700mA charging I.
Setting the reg to 13.8v will stop overcharging.
http://www.ti.com/lit/an/snva581/snva581.pdf shows some very simple ccts using the 317. You will need a transformer that can supply about 16/17vdc at the smoothing cap so the reg can do its job. A google on "LM317 battery charger" will find all sorts of circuits.

Good luck...
Mark.

[VK2OMD]

I'm hunting advice folks. I am going rucksack portable with an Elecraft K2. I was thinking of getting a gelcel battery for power. But I'm not really up to speed on recent battery technology.

A while back I asked a similar question about my FT817, and I was advised to use LiPo's. This is a slightly different scenario. Space and weight is not such an issue. I wont be walking very far and I will easily have space for, say, a 7AH gelcel. However, cost is an issue.

Battery and charger requirements are- inexpensive - easily sourced - simple.

What is the best way to charge a gelcel? I know this is a contentious issue and will likely result in multiple theories being put forward. But the keyword here is... inexpensive. I would like one of those digital smart chargers, but this project does not stretch to 100s of $$s.

The rig is about 1.5A on TX, 0.2 on RX, and I need to keep it on air for 3 to 4 hours. Does anyone have any good ideas?

A gel cell sounds the way to go, heavier than LiPo, but safer and cheaper.

I would NOT constant voltage charge it. There are lots of chargers around that suit gel cell batteries, and they are not real expensive. If you are home brewing, check that the designer limits current suitably when charging a flat battery, and tapers off at full charge. A lot of commercial chargers now use pulse charging for more rapid charge without overheating... but you didn't say whether recharge time was important.

One source of chargers might be those targetting the RC market. Sure, they can do LiPo, but lots will also do gel cells and can charge your 7AH quickly from a 12V supply (eg vehicle supply). Min you, many of these chargers make a lot of RFI. (http://www.hobbyking.com/hobbyking/stor ... UINE_.html)

Owen

[VK2AVR]

Gel cell you've got a couple choices, you can cycle charge it at 14.4 to 14.7V until the current falls below a certain threshold (3% of capacity as a guestimate so 7Ah battery stop charging it when the current drops to 200mA). Second choice if charge time is not an issue is float charge it for 24hrs at 13.5 to 13.8 volts. In both situations you want to keep the charge current below 0.2C (7Ah battery max current is 1.4 amps).

At the end of the day.. gel cel charging is like dancing on a pinhead. If you undercharge, the battery sulfates. If you overcharge, you get grid corrosion or electrolyte venting (if severe enough). The good news is that gel cells are so cheap that "near enough is good enough" and a cell will last you for a good few years. These batteries do eventually lose their ability to hold a charge over time, I had a 10 year old 18Ah battery that only managed 5Ah on a discharge test. 70% of the battery was dead weight! Toss it*, buy a new one.

* by "toss it" I mean recycle, obviously. Recycling lead acid batteries is a very worthwhile practice.


Also, if going for lithium batteries choose LiFePo4 cells. This chemistry has a perfect voltage for radio gear, a 4S LiFe pack will be 13.2 volts fully charged and will hold above 12 volts for most of their capacity. This chemistry is also more stable than LiPo or Li-Ion. Less chance of fire is good, right?

[VK2AXL]

Thanks for the info guys. That's great. Interesting that a 7AH gelcel will only give 1.4A max. Not a big issue as I'll be using SSB and most likely not at full power so I'll be well under 1.4A. I have also read somewhere that a gelcels lifespan is significantly shortened if it is completely discharged. So probably best to over estimate my charge requirements.

That link to the charger was interesting too. I notice they have others for under $15 which would be ample for my needs. Definitely within budget for this project. Charge time is not an issue. Doesn't matter if it takes days. RF noisy chargers aren't a big deal either, were it not for the fact I refuse to RF pollute on a matter of principle I'll have to deal with that issue when I come to it.

The LiFePo4 cells are very interesting. Though possibly out of my budget.

Does anyone have any links to gelcel suppliers? So I can get an idea of price, size, weight? I've had a bit of a google around and I'm finding it hard (or google is finding it hard) turning up suitably informative budget suppliers.

I'm planning doing a SOTA summit activation or two. Due to a couple of old injuries my days of long bush trecks are behind me, but as the rules say I only have to cover the last vertical 25m by foot there are a few local summits where I can park the truck half way up and walk the rest. That will be within the rules and should be within the scope of my unreliable knee joints. I don't want to blow a pile of cash on the battery-and-charger project as my first activation may be my last.

[VK3APW]

Hi Jack,
have a look at Hobby King, they have LiFe packs for model planes and cars, they have reasonably priced cell balancing chargers also. You can get 3S and 4S packs which are much lighter etc than gel cells. Silicon Chip published a low battery cut out project, last month, which would be ideal for protecting any battery source, Jaycar will have the kits soon I believe. Good luck with your Sota activations.

Cheers

Peter vk3apw

[VK2OMD]

Jack, you started off by making cost a prime issue. That will take you down the gel cell path.

I have written some notes on LiPo for ham radio at http://www.vk1od.net/equipment/LiPower/index.htm .

A current project is an intelligent battery monitor that has reverse polarity protection and low voltage cutoff... with low current drain and low voltage drop (one option is a latching relay).

However, I note that SIlicon Chip published a somewhat similar project... however it would want to be better based than the nonsense rolled out in the collinear antenna article to be worth trying.

Reason for this is that overdischarging LiPo is damaging.

Where did you get your peak current rating for the 7Ah gelcell? Seems pessimistic to me... I regularly use one for a sprayer at 4A and it gives good capacity and good battery life... you may have been listening to ham folk lore, there is plenty of it.

BTW, a friend ran a IC7410 on SSB at 100W PEP in QSO for a quarter hour off my 4200mAh LiFe battery and it used about 1/3 of the battery's capacity... the key thing is that it provided near 20A repetetive peak current safely.

Owen

PS: The Yuasa datasheet for a 7AH SLA battery lists the max discharge current at 40A or more, 1hr capacity at 4AH.

[VK2AVR]

Interesting that a 7AH gelcel will only give 1.4A max.

nonononono, that's the charge rating! 0.2C charge rate = 1.4 amps. You can discharge them quicker than that, the Jaycar 7Ah model specs a 40 amp, 5 second discharge limit. Derate that by whatever algorithm suits you for SSB duty cycles

http://www.jaycar.com.au/productView.asp?ID=SB2486

you can probably do better than 30 bucks if you hunt around.

Do not go below 75% discharge or the battery life will suffer. 75% discharge is perfectly fine for occasional use, as long as you charge it up again soon afterwards (lead acid does not like being flat, it wants to be stored fully charged and topped up every 6 months). The other metric I've seen is never go below 10.8V under load.

[VK2AXL]

nonononono, that's the charge rating! 0.2C charge rate = 1.4 amps.

Ah yes of course. Pardon my misunderstanding

I'll drop into the local Jaycar tomorrow and question the till monkeys.

Thanks all.

[VK2EM]

G'day Jack

I made one (actually two) of these...

http://www.qsl.net/wn2a/SLAcharger.pdf

Works quite well...

[VK2AXL]

To warp this thread up....

I got a 7AH gelcel from Jaycar. $30

And for charging I opted to take a more expensive route than first envisaged. I have bought a 7A conditioning smart charger for lead/acid batteries, which is also switchable for gelcels and a few other types of batteries. I have a truck which is rarely driven and the battery suffers from lack of use. The only lead/acid chargers I have are big output. One 200A and one 50A. So I have been thinking of a lower output conditioning type charger for ages. One I can leave hooked up and it'll switch itself in once in a while and keep the battery topped up ready to go.

So when I saw one with gelcel capability too, it was a case of two birds with one stone. And allowed me to mentally justify the expense by shifting the cost to the motoring budget rather than the radio budget.

Yes... pedantic... is probably the word