Portable Battery
Warnings
- Although we are dealing with low voltages, the energy stored in the batteries can provide enough current to melt wires, cause blinding flashes and fire.
- Do not attempt your own low voltage wiring unless you are electronically and mechanically capable.
Disclaimer:
- This is only a guide from my research, knowledge and experience.
- Please understand your own capabilities;
- Do not guess if you don’t know;
- Research & check the information before embarking on any battery project;
- If you have doubts about your own skills or knowledge, then don’t do it yourself;
- Use this information only as a guide in setting up your portable battery system.
Is a 12volt battery really 12volts?
- The 12 Volt (or 12V) battery has a ‘nominal’ voltage of 12. It’s not a regulated power supply of exactly 12 volts.
- The AGM (Absorbent Glass Mat) battery is the traditional type of Lead Acid battery used for a portable astronomy setup.
- The AGM battery may be between about 10.5 (fully discharged) to 13.8(float) (depending on the manufacturer) and about 14.4V when being charged.
- Components designed to be 12V battery powered usually operate a bit over 13V.
AGM battery
- AGM batteries should not be discharged below 50% of their capacity to provide a reasonable life expectancy.
- When the AGM battery drops to 12 volts it should be charged as 12V would be considered flat.
- AGM batteries, while initially cheaper to purchase, have a readily available alternative using Lithium that offer advantages of being lighter, smaller, and cheaper over the life of the battery.
Lithium Battery
- The Lithium battery technology that more closely matches the lead acid battery voltages, as well as being readily available because of it’s use in the Caravan industry, is the Lithium Iron Phosphate (LiFePO4 or LFP) (see Battery University)
- Don’t confuse ‘Lithium Iron Phosphate’ (LiFePO4) with the more generic term of ‘Lithium Ion’ which are batteries based on Lithium. (Iron not Ion)
- LiFePO4 batteries are not the same at the Lithium batteries in your mobile or camera. The Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC) is a popular choice for mobile phone and camera batteries. LiFePO4 are less likely to catch fire when compared to LiNiMnCoO2 batteries.
Lithium Iron Phosphate Battery (LiFePO4)
- LiFePO4 batteries are about a third of the weight of AGM batteries for the same AH capacity.
- LiFePO4 batteries while having a high initial purchase price, cost less over their life. AGM batteries need to be replaced more often due to lower number of charge / discharge cycles which ends up costing more in the long run.
- LiFePO4 can be charge much quicker than AGM batteries.
- LiFePO4 can be discharged to around 80% rather than the 50% recommended for the AGM battery.
- LiFePO4 batteries are the recommended type of portable Astro battery.
Portable Power System Size
An Astrophotography system should have the capacity to:
- supply the maximin instantaneous current needed with minimum voltage drop to all connected equipment;
- provide the power needed for the maximum imaging hours used;
- allow extra storage capacity for astrophotography system growth over time;
- provide the extra capacity for out of the ordinary use (drain with heavy dew conditions or slewing around the sky to more objects than usual or higher camera cooler current draw under warmer conditions);
- allow for reduced battery capacity with usage over time (80% of original capacity).
Other impacts on the Battery Capacity
- High humidity combined with winter temperatures, could mean that your dew controller will use more power during the night.
- Longer nights in winter results in longer imaging time using more power.
- More power is needed in the summer to keep the sensor cold, but less power is needed in the winter.
- Less power is needed if the sensor is kept warmer. Example: -10 deg C in place of -20 deg C (probably with minimal sensor noise difference between -10 & -20)
- Operating in colder temperatures will reduce the available battery capacity.
Battery Size Fudge Factors
- LiFePO4 batteries provide less capacity in the cold (allow about 15% extra)
- LiFePO4 batteries provide fewer discharge/charge cycles in the heat
- Batteries will provide more charge / discharge cycles for a lower depth of discharge (DOD):
- Target no more than 70% DOD or even less for longer life;
- It’s also best to recharge after every usage to keep each DOD small.
Voltage Drop
- A voltage drop in the wire results in a lower voltage being supplied to the equipment.
- The voltage drop in your wires could impact the operation of your equipment by not providing a high enough voltage.
- The large voltage drop from larger loads such as motors slewing on a mount would be reflected in equipment using the same wires.
- Battery setups and associated wiring design should minimise the voltage drop from the battery to the equipment.
- The Voltage drop along a wire equals the voltage at the start of the wire minus voltage at the end of the wire.
- The thinner the wire, the greater the resistance in the wire.
- The longer the wire, the greater the resistance in the wire
- The greater the resistance along the wire, the greater voltage drop along the wire.
- Minimise the voltage drop from the battery to the equipment with shorter and thicker wires. (such as placing the low voltage source such as the battery at the base of the equipment being powered).
Cables for Astro gear
The cables used should be able to carry the current to the device with minimum voltage drop. (don’t focus on the manufacturer's claims of current carrying capacity).
Example cable sizes:
- 6 AWG to a higher current device such as a small 240v AC Inverter;
- 8 AWG from battery to power distribution;
- 14 AWG for higher current devices;
- 16 or 18 AWG for lower current devices.
Resources:Cable size calculators
https://www.fabhabs.com/dc-cable-sizing-calculator
https://enerdrive.com.au/2018/02/06/choosing-the-correct-cable-size-for-a-dc-circuit/
https://www.wirebarn.com/Wire-Calculator-_ep_41.html
Battery Pack claims
Look closely for marketing tricks when comparing battery capacity.
- 10,000mAH (or milli AH) is just 10AH but it sounds big to someone not familiar with these numbers.
- A 42AH (or claimed 42,000mAH) at 3.7V is 155.4 Watt hours. BUT, at 12V it’s 13 Watt hours & at 13.8V, it’s 11 Watt hours. (As a comparison to this commonly sold 155.4 Watt hour Astro battery box, would need more like 1,500 to 2000 Watt hour setup)
''(as a side note on battery packs, if you need 240V AC out of a battery pack, make sure it specifies a pure sine wave output and not a modified sine wave output)
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Some more things to consider when comparing battery packs:
- Some portable battery packs on the market use higher voltage internal batteries. This typically could be due to using NMC batteries that have a higher voltage or large battery voltages inside the battery pack due to efficiency reasons with associated equipment. These packs then need to regulate their output to 12Vto provides 12V outlets because of these higher internal voltages. So, marketingfor the battery pack see this as a plus and highlight the 12V regulated output(s) claim in advertising often leading people to think they need a regulated 12V output for their battery, even though they are using a normal battery with 13.8V output. But is it exactly 12V or a more desirable voltage of over 13V.
- 12V is too low a voltage for your supply if you have voltage drop in your wiring (all wiring has a voltage drop to some extent).It’s not unusual to see astronomy cabling dropping the voltage arriving at the equipment of more than 1V for the thirstier types of equipment.
Battery monitoring options
- Guessing the Astro gear’s power requirements and guessing the battery size with no monitoring of the battery state of charge. The "fingers crossed, hoping it will survive" approach (the Politian's anti-science guestimate solution).
- Calculating the Astro gear power requirements and providing a very conservative battery capacity with room to spare as the battery ages. Additionally, ensure the battery is fully charged before each Astro imaging session. No monitoring of the battery state of charge as the sizing has been over-provisioned.
- Calculating the Astro gear power requirements and providing a conservative battery capacity, while monitoring the live battery capacity over time to ensure the system really can provide power for a night of imaging. (this is the recommended method)
Battery Box
LiFePO4 does not have the potential to leak as does the Lead Acid variants so does not need a battery box. Just ensure the battery is appropriately protected from damage.
A construction option is to choose an appropriate battery size with the minimum hardware attached (fuse and connecting cable and plug). The battery should be protected from knocks as appropriate to how it’s used.
A ready-made battery box solution can be used but they typically are compromised in some form to manage costs. They may use smaller chargers, possibly unnecessary components like inverters and poor connecters such as cigarette lighter plugs.
Fuses
- Fuses are needed to protect the wiring down stream from the power source and not the device. They are placed as close to the source of the power as possible. This will ensure the minimum amount of cable is left unprotected before the fuse.
- Fuses are placed in the positive side of the battery wiring.
- Fuse types should be selected for minimum voltage drop across the fuse at the rated current being carried.
- Fuses protect against:
Short circuit – when one conductor of the battery comes in contact with the other conductor forming a current path back to the battery.
Severe overload – the cable carrying more current than designed due to a short circuit or faulty device.
- The size of the fuse needs to be small enough to blow before the wire exceeds its current handling capability, but large enough to handle the current used by the device being powered.
- Where cables change thickness between the power source and the device, the fuse should be sized to protect the thinnest cable in the circuit before the next fuse.
- The type of fuse (or fuse construction) should be suitable for the current being carried. Example: Inline Midi fuses for the higher current feeds and auto types for low current feed.
Equipment power distribution
- The key to distribution of power from the battery is providing sufficient power to devices with minimal voltage drop and safe wiring.
- I would argue against the need for multiple batteries by separating the higher current drain devices like mounts or dew control. Power problems from motors can be avoided by using correctly sized short cables. Flat batteries caused by dew heaters can be avoided by correctly sizing a battery.Use direct cable runs back to the battery for higher current devices (motors and Dew Control)Higher current devices will cause issues if the cabling is thinner and/or longer, rather than using the recommended thicker cable sizes and shorter cable lengths.