Application Note No. 47899.13 Batteries de W3IY updated 08-11-02
If you are going to go out roving, you may need some serious batteries. Depending on your desired RF output levels, you will probably want to use a lead-acid deep cycle battery. A TE-Systems 1452G (375W output) 2meter amplifier will draw around 70A, peak current...35A average on transmit. Even a 100W rig will probably pull down a fully charged 100 amp-hr battery in a couple hours of contesting, if the temperature is below 60deg F or so. (Don't forget about all that "other" equipment you have continuously draining the battery). If it's cold, you will be lucky to get 1 hour if you call CQ a lot. Deep-cycle batteries are designed to be used-up and then recharged. A normal car battery is not designed for this type of abuse, and won't last for too many charging cycles, if you run it down consistently (like maybe 10 times, max!!). They are designed to store charge, and deliver high peak currents required by your cold engine on a winter morning. You can get deep cycle batteries at places like Wal-Mart, K-Mart, etc. I don't really know how good or bad these are, compared to other types (probably marginal...but cheap). I have read about professional type deep-cycle batteries where the cells are spiral-wound plates structures. These are supposed to be the best type, but I suspect that they cost more $$ also. Before you put extra batteries in your vehicle, read up on battery safety. Batteries can explode since hydrogen gas is released while charging. (I once heard a story about a guy that was having fun-showing off by drawing huge sparks off of a charged battery. It blew-up, covering him with sulfuric acid. He was not physically injured (luckily), but as he walked home to change clothes and clean up, his clothes slowly fell from his body as the acid ate thru the cloth!! He was almost indecent when he got home). It is very easy to cause an accidental spark while working on batteries with charger cables, etc. Always be sure your charger/engine is off when you attach the cables to the battery.
Here are some guidelines on batteries.
1) Put your batteries in a water-proof box, so the acid and acid vapor won't hurt stuff (yes, even the gas will eat things).
2) Make sure you have plenty of ventilation when charging the batteries (maybe leave windows open, etc). After charging, air the place out well before energizing any equipment.
3) Get or build a good battery charger. A constant-current charger is good, except that the voltage needs to be raised after 80% charge is reached. I use my Astron 50A adjustable current unit sometimes, but a full charge is hard to get this way. (rovers don't usually have time to get a full charge on the road...hi). Try to get a charger that tapers off to a trickle charge over the charge cycle.
4) Try to carry a spare battery in case you run down the vehicle battery (assuming you are using it for radios). Always have a back-up vehicle starting capability.
5) Don't forget that even if you go out with really BIG batteries, they will eventually need to be charged. Doing this correctly will cost more than the batteries (unless you are resourceful). You can't just hook them up to your alternator, unless you have a high-performance alternator. This can be a pretty expensive piece of equipment. I found a pretty decent-reasonably-priced unit from www.mralternator.com in SC. It is the same size as the original 100A Delco unit, and it boasts 160A service for $179.00. This unit really performed well in the June 2002 VHF contest. While driving, it nicely supplied all the current for the radios (>70A, peak), and while I wasn't talking, it kept the battery string charging up. It seems to deliver full current, even while idling, due to a favorable pulley ratio. Even when my batteries got low, the alternator current remained at a reasonable 40A or so. It was surprising to learn that my engine draws almost 20A just going down the road, with radios turned off!
The bottom line here is that BIG batteries alone probably won't get you through a contest, unless you take care recharging them...switching off between banks, and not letting the batteries run down too much. This way, they won't suck too much current from your charger/alternator, and overheat it. When you overheat an alternator, the diodes come unsoldered, and leave you stranded out in the boonies. Also, never disconnect your batteries from an alternator with the engine running. This will cause voltage spikes which will also trash the alternator. We have many lessons to learn here from the RV industry.
6) Always bring jumper cables into the field. If you wind up using them to hook up stuff, wrap them in insulating cloth, or something. Sparkz is BAD. Hydrogen from your batteries will explode if a spark gets near. Check your battery connections carefully. If you get oxidized terminals (white powder on them), STOP and clean things off. Intermittant connections are prone to sparking, when you suddenly place a heavy current through them. A friend of mine recently blew up an electric lawn mower. It was freshly charged, and all he did was turn it on. Make sure to vent the hydrogen gas out of the area before firing up equipment...especially if it's an electric motor, which generates sparks (all of them do this!). An exploding battery may not blow up your vehicle, but it really sucks to clean sulfuric acid off of expensive radio gear. (and the rags will disintegrate as you do this...and here you thought acid-core solder was bad for electronics...Yipes!!).
7) Use heavy wire for safety and for higher performance. Most ham transmitters work very poorly if you have voltage drop in your wiring. At 20 Amps, it doesn't take a very long run to loose a few volts. I find that my IC-706MKII is really sensitive to low voltage...I start getting horrible audio reports on SSB, if the supply voltage gets barely less than 12VDC. The FT-100, on the other hand, simply starts jumping several KHz in frequency, but not until the voltage gets much lower.
8) Consider adding a battery isolator to your rover-mobile. This will prevent you from sucking down the main starting battery. Some isolators are available which have very low voltage drop. The cheaper ones are just big diodes...giving around 1.0 VDC drop at full load. This is tolerable, if you use a Jacobs-box or equivalent DC-DC converter to maintain full clean voltage at the equipment (see http://www.jacobselectronics.com/products/caraudio/fr1500.htm for details on a 100-amp unit. They also have smaller ones). Power amplifiers are famous for putting out less-than-advertised RF power at lower voltages than +13.8VDC. You won't see this voltage unless you are connected to an alternator, with the engine running, or-unless you use a Jacobs box, or equivalent.
Another idea is to just use a heavy duty battery switch to connect your batteries to the vehicle alternator.. West Marine sells nice units that handle 300A. Don't forget to turn off the switch while your getting some sleep, though, as you make awake to a discharged vehicle battery. This will cause you to loose precious time while roving, or worse! (you did bring extra batteries & jumper cables, right?).
9) Always keep a trickle charger on your batteries when you aren't using them. A winter spent in the cold garage is very BAD for battery life. Most Important is to always charge your batteries immediately after using them. A discharged battery will "sulfate" if it sits around. This is when you look into the holes and see white powder all over the place. The powder is sulfur, having been plated out onto the lead plates, insulating them, and making them rather useless. It is possible to reclaim a sulfated battery, but expensive equipment is generally required.
10) Consider using gell-cells. These are much safer, and pack a pretty good charge capability. They cost a little more, but may save you time and trouble.
11) Always fuse your radio gear. A charged battery will vaporize wiring if there is a short, and can cause a FIRE!. This is VERY BAD! (NiCd batteries can be particularly lethal if you short them out. When shorted, even small NiCd batteries will really deliver the juice, until things go up in smoke!!!....(trust me...I have seen this in practice!). Lead-acid batteries can also be quite dangerous.
12) Carry a fire extinguisher if you have large batteries running loose in your rover vehicle.
13) From what I have read, you should never charge your batteries at more than 40% C (even this is really pushing it...many manufacturers recommend 20% C). If you charge too fast, you will need to add distilled water often (especially in warm weather). For a Wal-Mart big deep cycle unit (~100Amp-hours), max current is around 40A. What this means is that you will need 2.5 hrs charge time to get them back up to the 75% level. Getting the last 25% charge takes longer, and requires tapering off the current. You can probably get away with faster charging, but battery life will suffer. (ah, yes...the problems you run into roving). The best solution is probably to 1) bring extra batteries, 2) don't call CQ all day with high power, 3) try to find yourself a motor generator set that you can put to work on your lowest battery while you are busy discharging another one. If you do this, you will need a 40A power supply or charger. I have used an Astron RS35M, and it gets hot, but hasn't melted as yet. 4) consider the merits of QRP operation, 5) Consider running your gear from the AC off the generator, and leave the batteries home. You may need to check with the rangers at various parks and other places to make sure they allow gasoline engines. Some parks have fire restrictions and noise ordinances which could get you busted. At least some parks require "flame-arrestors" on generators.
14) Don't forget about "power-factor" when you are figuring out your AC power requirements. A typical Astron 35A power supply will need around 9-10Amps at 110 VAC, since it is not a resistive load. When a reactive load (like a transformer with a capacitive input rectifier filter) is connected to an AC power source, the current is not in-phase with the voltage, and the result is that you will need to supply additional current for a required amount of power. Many linear power supplies are only 30% efficient (apparent) at converting AC current into DC power.
15) Learn how to read your battery voltage to determine charge state. 12.6VDC is a full charge for lead-acid batteries.
I recently learned about some really awesome batteries. The Trojan T105 battery is legendary for high-performance batteries. It is a 6V 225 Amp-Hr unit which has achieved an incredible reputation through years of research. The 6V design provides extremely rugged-thick plates. Put 2 in series for a super-rover battery system. They cost around $70 each, and are a real bargain. These puppies were designed to run golf carts! Check http://www.trojan-battery.com/GolfDC.asp?Product=51 for details.
16) If you are considering one of those cute little Honda generators with a built-in inverter, BEWARE the RFI that they make. I spent a lot of effort filtering the noisy output, only to discover that even the generator by itself makes lots of noise on 2m. I pulled the AC cord out, and still had objectionable RFI from the unit. The inverter is a really nice feature,which always gives you 60 Hz power, even at low RPM (saves fuel), but it does make undesirable interference on VHF bands (probably worse on 10m).
I have also run into problems with the EU1000i generator shutting itself off. I have it mounted to the rear bumper, and it appears to have sensitive shut-down sensors to detect low oil levels and stuff. I think the vibration while driving triggers these sensors, and keeps the unit shutting off. Maybe I will have better success with less oil in the thing...need to test this (within reason, of course...I think I may be slightly over-filled). I have since found that my heavy-duty alternator can charge batteries just fine, so I probably don't need the Honda generator, as bad as I thought I did. I just don't like to over-tax the alternator. Things that get too hot to touch should probably be given a rest every now and then.
Anyways, check-out Battery Info for more information. They have a good tutorial on different types of batteries.
Don't get caught out roving with dead batteries!
Be SAFE with those high DC currents!
73 de W3IY/R