Larry & Others Larry has given us some very good stuff below to consider and to take into account.
I would like to add just a little bit more to it especially in reference to the LiFePO4 (Lithium) batteries that are very prevalent these days. Nothing against LiFePO4 batteries it’s just that you need to understand how they work, more so than the conventional lead acid battery. When sizing a battery for your aircraft and application you would normally size it based on a few things. 1. It ability to provide enough amperage to turn the engine over from a cold start (in North America this can be very cold), you also want a few attempts at cranking the engine. Before the battery is flat. 2. The physical size (& weight) of the battery and where it is going to be fitted within the aircraft. 3. Thirdly (as Larry mention below) the capacity of the battery. 4. Expected battery life Just a side note on battery life, in most batteries, life of a battery is determined by Depth Of Discharge (DOD) and quite often is quoted in cycles to a DOD of X%. A battery may be quoted with a life of 5000 cycles to a DOD of 10%, or (the same battery) may be quoted at 1000 cycles at a DOD of 50%. A ‘cycle’ is; one discharge and recharge event to a DOD of X% (whatever DOD is quoted). In short what this means is, if you have a larger than necessary battery you can expect a longer life than if it is a small than necessary battery. What is the necessary size of battery you need? It is determined by those four points above. Electrical Load calculations As Larry has stated below you need to do some load analysis on your aircraft electrical system. Understand how long you have left in the air if the alternator goes belly up (Kaputt). This is based on the batteries Amphour rate(Ah), and you would do this calculation based on 80% of the batteries NEW specified amphour (Ah) rate. Eg. if a NEW batteries specified Ah rate was 100Ah, you would base your electrical load calculations on battery capacity of 80Ah. Most battery manufacturers consider their batteries are at End Of Life (EOL) when a battery can only give out 80% of its rated NEW capacity. So your calculation should be based on this 80% of its specified NEW Ah rate figure. As Larry has pointed out if you have all the wiz bang electronic fuel/ignition system they can be drawing a significant amount of current. Add to this if you also fly with electronic flight instrument system (EFIS) you could be drawing 15 to 20 amps (maybe more) continuously while you are flying. If you have these types of goodies in your aircraft you need to do an electrical load analysis on your aircraft. Old steam gauges (conventional flight gauges & a magneto system) were usually very power efficient, a few amps while flying at most. Keep in mind if you have an 100Ah battery, it can supply 1 amp for 100 hours, or it can supply 10 amps for 10 hours, or 100 amps for 1 hour. In reality a battery manufacturer will usually quote their batteries at the 10 hour rate, as not many battery will give 100 amps for 1 hour. Another important point about LiFePO4 batteries they have a slightly higher operating voltage, which is good for your aircraft, especially when starting. However if you do a direct replacement of you OLD lead acid battery without consideration of the voltage indication and warnings (eg. low voltage warning lights and meters). The (LiFePO4) battery can give the impression that things in the aircraft a rosier than what they really are. Also one other very important point to seriously consider, lead acid (old technology) batteries just kept on giving and would show lots of signs becoming flat (lights going dim, voltage meter readings low etc.) but they would keep on giving till dead flat. With LiFePO4 battery they have a Battery Management System (BMS) and these will cut off the power from the battery at a predetermined point with usually NO warning. The voltage difference between fully charged and flat (cut off) is not like lead acid batteries. The voltage range between charged and flat is a lot closer. So if you have a LiFePO4 battery (or thinking of getting one) consider very carefully your electrical load while flying and have a plan of action if your alternator dies. Or at least know how long you have before the battery is switched off by the BMS. And you have nothing after that (if you only have one battery system) eg. the lights really go out. One other thing to consider as well, is where you fit your LiFePO4 battery. Lead acid batteries were/are very tolerant of heat, their capacity would vary with internal temperature but on the whole very tolerant of heat. LiFePO4 are not as tolerant of heat and should really be outside of the engine bay. Flying probably not so much a problem, plenty of cooling air, however heat soak usually just after landing on a hot summers day can be very high and it can damage the battery. Maybe the engine bay is not the place for a LiFePO4 battery. Hope you find this useful. Pete Leonard From: KRnet [mailto:krnet-boun...@list.krnet.org] On Behalf Of Larry Flesner via KRnet Sent: Saturday, 18 May 2024 4:04 AM To: krnet@list.krnet.org Cc: Larry Flesner <fles...@frontier.com> Subject: KRnet> Batteries, more final thoughts One thing that was not really discussed in the "battery" posts was "reserve capacity". After starting power (CCA - cold cranking amps) , reserve capacity is #2 in importance. The engine started fine but then what happens when the alternator / charging system fails. If you are running electronic ignition, flat screen panel, radios, GPS, cockpit lights, charging your cell phone, and more, how low will the battery last? Reserve capacity explained: To calculate the reserve capacity of a battery, it must first be fully charged. Manufacturers draw 25 amps of power from the battery at 80°F. When the voltage drops below 10.5 volts, they stop drawing power. The duration of time this process takes is the battery's reserve capacity, which is measured in minutes. The PC680 is a very popular battery in experimentals. Here are some of it's numbers. OEM Part Number PC680 Voltage 12 Volt Cold Cranking Amps rating at -18°C (0°F) 170 Reserve Capacity 25Amp Draw 24 Whatever battery you are using or intend to use, find out what the "reserve capacity" is, determine what systems you will need to make a safe landing, calculate the total draw in amps of those systems, and now you can calculate and know in advance how many minutes you have to find a safe landing site. Allow a safety margin. The more critical electronic systems you have, i.e. electronic ignition, electric fuel pumps, the greater your need to identify a charging system failure when it happens and know immediately what systems you can shut down and how many minutes you have to find a safe landing site. Suggestion: Have a warning system (idiot light) to identify charging system failure and a placard saying "land in X minutes". Also, realize that "reserve capacity" deteriorates with battery age. Did the starter "hesitate" on the last start up? Is the battery fully charged or did you have to jump start on the last startup? None of this is rocket science. Get the info and know the answers before it happens so you can avoid the panic and survive to tell us all about your near death experience. 🙂 "There I was, above a cloud deck at 10K and d#m*, the alternator fail light came on, bright as the sun!!!!! If you survive you are allowed to embellish the story a bit as most of us are inclined to do. Larry Flesner
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