< But what a difference there is between the two technologies for equivalent capacity batteries in terms of the weight you have to carry and the space taken up in your rucksack!
Undeniably so John. There is also the issue of cost which has been mentioned. I personally use SLABs as they allow me to use a small linear after the 817 which hopefully helps on marginal paths, they cost not a lot and being a Uniques activator I regularly use (to exhaustion) several batteries in a day. If I needed to fund 4 LiPo batteries, something else would not have been purchased.
You pays your money, you gets what you gets - in my case weight! I also carry bothy bag, survival kit, first aid kit, etc. The weight of the SLAB is only part of my total load and I am happy to bear it.
The debate seems to have come down to a comparison between LiPo and SLA technologies. Not much has been said about NiMH which seems to me to offer a good solution in terms of weight vs cost. After all, guru Richard G4ERP uses them and he is extremely weight conscious.
Brian: Tee hee! I ain’t no Russian hit man! Perhaps I should call 'em Li-Poly batteries instead.
Gerald: Yes, of course. The debate was about usability rather than cost but yes, Li-Poly batteries are somewhat more expensive. Ya pays yer money and takes yer choice.
I didn’t have very good experiences with NiMH batteries. I think that’s because I was trying to use multiple primary cells and was losing quite a lot in all those springs and wires that make up a battery holder. Absent that problem I expect they make an adequate power source that sits somewhere between the SLA and Li-Poly solutions in terms of cost, weight and usability.
I didn’t have very good experiences with NiMH batteries. I think
that’s because I was trying to use multiple primary cells and was
losing quite a lot in all those springs and wires that make up a
battery holder. Absent that problem I expect they make an adequate
power source that sits somewhere between the SLA and Li-Poly solutions
in terms of cost, weight and usability.
73, John.
I found the same problem with NiMH cells John, until I switched to the ones with solder tags. You can make up a pack at whatever voltage and current rating suits you, then shrink wrap the pack to make a very reliable and sturdy pack at reasonable cost and weight.
This reminds me of an idea I had of using an ammo belt for holding a number of small 12v SLABs I had or a shotgun belt with NiMH C cells wired together. I thought it was a great idea as it would spread the weight evenly until my XYL pointed out that I was likely to get my self shot as a suspected terrorist. Tee Hee!
In reply to G8ADD:
Hi Brian, I have just bought some 2700 mA NiMHs cells that are on offer at Maplin and I am thinking of undertaking the “green-wire mod”. Do I understand correctly from what you’ve said, that you can now charge this pack with the standard charger and without any further hard-ware mods? I guess you just need to extend the charge time as per the hand-book or do you have a recomendation based on your experience?
My 817 has the “green wire mod” and an AA battery holder. It will charge NiMH/NiCd cells. The downside is that the charge current was set for the old original NiMH pack and that is not enough to charge a large capacity pack in the 10 or 6 hour periods. You can do several charge cycles to fully charge higher capacity packs.
From years of experience, series charging of cells tends to be worse than parallel charging. You cannot guarantee the discharge rate of the cells in series and eventually you end up overcharging some cells when you series charge. I invested in a smart cell charger which charges AA cells individually. It can cope with 6 AA cells at a time. You do have to remove the cells from the 817 to use it and it takes some effort to ensure you charge all 8 cells and don’t forget the last 2 after doing 6 cells! I think it was £15 from Aldi/Lidl when they had AA 2500mAhr cells on offer.
In reply to MM0FMF:
Andy - many thanks for your comments on charging with the 817 - and I fully agree with your comments on the serial charging of cells. I think I’ll invest in a charger but in the meantime it’s useful to know that the 817 will just about cope if and when required. I’ve been reading the general thread on batteries with great interest, however I’ll continue to use my 7Ah Slab as the main power source as I intend to run a 25watt PA as well. Also re-charging “on the move” is easily undertaken from the car acc socket. Thanks again.
In reply to M0COP:
Hi Pete, I have a lead which is fused in both the -+ legs and a diode to prevent the Slab feeding back and discharging back into the vehicle supply. It appears to work so far and appears fine for topping up the battery between hills. I then put it through a discharge / charge routine when I get home, using an intelligent charger.
Is some sort of interface needed to charge a SLAB from the car accessory socket?
Not really. A 12V SLAB is just a little car battery. Fuses are a good idea. Some acc sockets are switched with the ignition. The front one on my car is switched but the one in the luggage area isn’t.
If you are going to use a diode (not a bad plan) use a schottky type, they have less forward voltage drop than conventional PN silicon types. They’re used with solar panels for this reason.
Battery manufacturers generally recommend that you don’t discharge their batteries for the sake of it - you just use up battery life that could be used to power your radios! Particularly in the case of series-connected cells serious damage can be done not by overcharging but by reverse-charging of the weaker cells in the battery. Lots of info on the manufacturer’s web sites.
In reply to MM0FMF:
Hi Andy, an interesting concept which hadn’t crossed my mind was charging an SLA from the car. The only concern I have is limiting the current. I have a 3.3Ah SLA and the maximum stated charge current is 990 ma. With the lead you describe there is nothing to limit the current, other than installing a fuse that would blow if the maximum charge current is exceeded.
I suspect if the battery is only partially discharged then it may not draw much current to top up, however if it is fairly discharged this may be an issue. Not a 100% on this, its just my understanding of how they should be charged.
The simple way to get around this is to not connect a well discharged SLA to the car and only ever charge it from a charger with a current limit circuit.
High currents are not good. High currents for a long time are worse. You may want to only consider it as a method of last resort and not the normal means to recharge.
There again, I charge my SLA by slapping it across a lab PSU set to about 14.2V and as long as the initial current is under 2A I don’t loose any sleep. It’ll be down to hundreds of mA soon enough. This cell has done about 80 such charge cycles and is going OK. At £10 replacement, they’re not expensive enough to worry too much. I must admit that I keep an eye on the current just to make sure it’s dropping with time which you can’t do with a straight lead into the acc socket.
In reply to G3WGV:
With 3 cell LiPo batteries what charging current should I select? I have a new GT Power A606 Dual Charger that I’m learning to use. There is this mysterious phrase of “charge at 1C”. One battery is 4000 mAh and two new ones are rated 3300mAh. I think the answer is around 2 amps. I’m holding back from doing the first charges till I know. The Chinglish instructions are unclear on this point.
I left a LiPO attached to a radio by accident last week. This week it was completely flat and, despite several efforts at revival, it has not recovered.