Paul has some good points about batteries. Here are a few more: A so called marine deep cycle/starting battery is nothing more than a little heavier starting battery. A true deep cycle battery has much larger plates than a starting battery.
A starting battery has thinner plates which allows it to release energy faster than batteries with thick plates. The thin plate batteries can release very high currents in a short time but too much and it will warp the plates. A true deep cycle battery with heavy plates can not supply current as fast as a regular battery but it can be discharged to a lower point without harm. A regular starting battery should not be discharged below around 80% and should be immediately recharged after the draw down. A deep cycle battery should not be discharged below 50% and should be immediately recharged after use. Every discharge will shorten the life a battery. Leaving it in the discharged state for a few days is sure death of the battery. Proper charging of batteries needs to be controlled. Too much charging current will warp the plates just like too much discharge current and ruin the battery. Batteries need to be overcharged a little so that there is some gassing or they will never reach full charge. Batteries also need to be floated at the proper voltage to maintain their life. 13.6 volts is about right for most deep cycle batteries and some regular automotive types. It depends on the material used in the plates. In most cases 13.8 float voltage will kill these batteries over time. A tenth of a volt or so is critical with the float voltage. AGM batteries require a different float voltage. Ask the manufacturer what it should be. A battery that is designed to be floated at 13.6 volts will never reach full charge at 13.6 volts after a discharge. It will take at least 14 volts to charge it properly with current limiting. There are a lot of old 13.8 volt chargers out there that have been around for years. These were used a lot on boats. They were big heavy beasts that could supply lots of current when needed. Most were the fero-resonant regulated type. 13.8 was a compromise between charge and float voltage. They are responsible for more battery sales than any other cause. 13.8 will not fully charge the battery but at the same time it will allow it to gas and boil off the water over time which exposes the plates and kills the battery. If the batteries were floated at 13.6 they would last a long time but will not fully charge at this voltage unless they were first charged at a higher voltage. Just because you have a 140 AH battery doesn't mean that you can draw that much power from it in an hour. That is usually a 10 or 20 hour ratting. That means that you could draw 14 amp from it for 10 hours. Then it would be dead. Since you don't want to go below 50% for a deep cycle battery that means that it would only last around 5 hours. Note that a 10 hour verses a 20 hour curve is different. You can't just half or double one to get the other. They are not linear. The 10.5 volt rate for a dead battery is with no load. A starting battery will drop down to 7 or 8 volts sometimes under starter load. It all depends upon the internal resistance of the battery. As they get old the internal resistance goes up as the plates become sulfated and they are not able to supply as much current without a large voltage drop. Think of it this way: If you need large amounts of current for a short time then a regular starting battery is best. If you need moderate amounts of current for a long time then a deep cycle battery is what to use. Find out what the recommended discharge rate is for the batteries that you are going to use for backup and match that against the current draw of your equipment. You can't just arbitrarily throw batteries in and expect good results. AGM batteries are not all they are cracked up to be. They are really just a regular flooded cell type battery but with a gel rather than liquid electrolyte. They have matting to hold the gel to the plates. Some may have a different plate material makeup. The big advantage is the no vent requirement. The best bang for the buck in deep cycle batteries are gulf cart batteries. Usually they come as 6 volt batteries. They are very tough, can withstand lots of vibration and abuse. Very good life if properly managed. You do need to vent liquid filled batteries to outside air. On boats when the batteries are mounted in the engine compartment they have natural venting from engine air vents. You don't want them in a sealed box. Placing these type batteries in an equipment cabinet may be fine for a long time as long as they are just being floated. But when the equipment goes down and a heavy draw is place on the batteries they will gas. Also during charging they will gas, sometimes heavily. If there is not much air movement for the gas to escape the cabinet an arc from a relay can set it off. Same reason that you always want to turn off a battery charger before disconnecting the charger leads from the battery. I have seen a couple of batteries explode when someone disconnected the charger from the battery first. It is a bomb! That's more than most will want to know. 73 Gary K4FMX > > As another note . . . > Please do not tie a back-up battery directly across the main output > terminals of a power supply (which provides no current limiting to the > battery while re-charging). I have found this actually done at a hill > top site. What had happened was that AC power to a repeater had been off > for three days or so. The repeater was running exclusively off the > battery for 3 days and finally depleted it. Well, when AC power was > restored, the battery started pulling an enormous amount of amperes from > the power supply (since it was now depleted). I was there at the time > the AC was restored, and within a matter of a minute of the power being > restored, I smelled something burning. After sniffing out it's point of > origin in the room, I found that the power supply tied to the battery > was too hot to touch, and that the wires connecting it to the battery > were just about melting the insulation off. What a fire hazard. I > disconnected that battery right then and there and notified the owner of > the repeater. So please don't burn down our repeater sites. It's hard > enough trying to get into commercial sites as it is. Many radio sites > will never again accept an Amateur Radio station due to previous > experiences with hams. Please don't add to that. > > Have Fun ! > > > Paul Metzger - K6EH > DVARA > > >
