Portable Power Solutions for Astronomers: Part 3, Doing it Again Tomorrow Night

In Part 2 of this series, we examined the how battery capacity and total device current draw over time relate to having enough power to get through a night of astronomical observing. In this installment, let us assume that we had enough battery capacity and/or our devices did not draw too much power, and we just got through one great night astronomy adventure. But now our battery is depleted and requires recharging before it can be used again. First, we will examine some battery charging basics and then we will consider options for charging, including some ways to view charging relative to time. And finally, we will suggest a way to assess whether your charger can get your battery ready for tomorrow night, i.e., using the battery for a second consecutive night after a charging period during daylight hours.

Some batteries can accept higher charging rates than others, so care must be taken when selecting a battery charger. Also, AGM and Gel batteries must be recharged using a charger that has the capability of charging AGM and Gel batteries. There is no substitute for reading the instructions that came with your specific battery.

Battery charging is a function of the same factors as battery depletion: battery capacity, charging amps, and time. Charging amps is the opposite of amps that draw down a battery’s charge. These amps are electrons, or charge, flowing back into the battery. As the battery’s amp hour capacity increases, more charging amps and/or more time are required to restore the charge. No matter the capacity, more charging amps result in less charging time to restore a battery to a full charge.

Consider a 4 amp charger and a 2 amp charger connected to two identical batteries at identical levels of discharge. The 4 amp charger will fully restore the charge to a battery twice as fast, or in half the time as a 2 amp charger.

With the relationship between amps and time covered, lets now vary battery capacity. Keeping the charging amps constant and varying the battery capacity, a 4 amp charger will recharge a fully depleted 17 AH battery in roughly half the time that it takes to recharge a fully depleted 33 AH battery.

A battery charger, once connected to a battery, will begin applying a charging current to a battery as soon as the charger is powered on and connected to a battery. Unless the charger is manually disconnected from the battery once a full charge is reached, the continued application of electrical charge will eventually damage the battery. Once the charger is disconnected from the battery, the battery’s charge will begin to slowly dissipate over time. If a long time has passed since the battery was on a charger, the user might find that its charge has significantly diminished upon trying to use it. The counter to this might be to reconnect the battery to the charger for a period to top off its charge just before use.

There is a special kind of battery charger that is commonly referred to as a smart charger. Other terms are for this type of charger are maintenance charger or float charger. A battery is more easily maintained with a smart charger that is rated for that type of battery. Once connected to a battery that is at less than full charge, they work just like an ordinary battery charger. When the battery reaches full charge is where the special capabilities of a smart charger come into play. The charging current is automatically stopped by the charger, and it then goes into a monitoring mode. At this time, the battery’s charge will begin to slowly dissipate over time as any other battery does when removed from a charger. When the battery’s charge has depleted to some specified level (usually just a small amount of depletion), the smart charger will automatically cycle back on until the battery’s charge is once again topped off.

This cycle of charge-monitor-charge will repeat for as long as the smart charger is connected. It works much in the way that a home heating system thermostat keeps the room temperature within a narrow range. Smart chargers have the advantages of being healthier for the battery as they prevent the battery from damage that may be caused by either overly discharging, or damage from being on a charger too long. A practical advantage from a use perspective is that a smart charger ensures that the battery is always fully charged and ready for a night’s work, but that only works to a point…

But consider a multi-night regional star party scenario. Say that on the first night most of the battery’s usable capacity has been depleted. It must go on a charger to get ready for the next night. Ideally, the battery will be completely recharged during the daylight hours. Depending upon the amount of battery capacity that is to be restored, and the charging amperage available from the charger, the daylight hours of a single day might not be enough time to fully restore the charge.

If the amount of charge that is to be restored is accepted as a constant, and the amount of time available to recharge the battery is also a given, then the only way to speed up the process is to apply more charging amps. Smart chargers have amperage ratings that range from well under one amp and up to five amps (or more) are commonly available. If the astronomer needs to restore the charge to a large capacity battery in a single day, then the higher rated charger is likely to be required. On the other hand, if the astronomer does not anticipate ever using the battery for two consecutive nights, then a lower rated charger is likely to suffice.

A good place to start when assessing the adequacy of a battery charger’s recharge time for a specific battery is the battery charger manual. Most are available online and will contain a section that shows the amount of time that the charger will require to recharge a battery based on the amp hour capacity.

Much in the same way that battery life rehearsals were done at home in Part 2, we can do charging rehearsals as well. It is just a matter of keeping a log regarding battery capacity, charging amps, and time. Below is a sample battery charging log. Unlike the prior log where an entry was made each time the load on the battery changed, this log has one entry per charging event. Some monitoring of the charger status lights is required to identify when a smart charger transitions from the charging mode (which indicates that the battery if fully charged) to the maintenance mode. Also, this example assumes that the astronomer owns two batteries, and a log entry is made each time a battery is charged.

Battery Unique NameCharging AmpsStart Date/TimeStart VoltageStop Date/TimeEnd Voltage
Kendrick 33 AH2.755/23/2020 083011.85/23/2020 163013.2
Kendrick 18 AH.755/23/2020 083012.45/23/2020 131513.2
Kendrick 33 AH2.755/24/2020 074512.25/24/2020 154513.2
Kendrick 33 AH2.756/18/2020 070012.25/24/2020 150013.2

Now that we have covered batteries and battery charging, we can move to Part 4 and consider the entire power solution. Helpful comments below are welcome.

© 2021 Jim Johnson and Doug Biernacki

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