Crawler Battery Considerations
There are three common types of batteries that can be used in inspection rovers. They are all rated for voltage, amp hour capacity, and “C” ratings (for discharge and charge). C ratings are important for power hungry drive trains in performance vehicles, but not so much for rock crawler adaptations and tanks, so I won’t get into those. What is important is capacity, and each battery type provides for a way to gauge that.
Obviously, we want to finish an inspection before the battery runs down, maybe even two. We need enough reserve power to ensure no system drops offline, enough energy to power the various continuous and intermittent loads onboard.
Major examples are …
Continuous: VTx, lights, fan (s), Rx, Tx Switches, regulators, camera (s)
Intermittent: Motor, servo (s),
A consideration is the minimum voltage needs of equipment. Many VTxs (video transmitters) will fall off below 7v. The vehicle’s Rx (radio control receiver) does not like running below 6 or 5 volts, depending on vintage. If you lose your video stream, getting back might be a problem. Lose your Rx, and the machine is dead.
Voltage x Amp/Hrs = WattsHrs. Watt Hour value is a good way to look at a battery’s suitability. Once you’ve selected a minimum voltage level, you can look at capacity, and then maybe increased voltage. When looking at voltage, though, consideration of the speed controller’s maximum is wise. They are directly connected to the battery, and have limits. Typically, staying below 12v is safe.
LiPo, NiMH, and LiFe (LiFePO4)
Each chemistry type has is advantages and disadvantages; the number of charge cycles before exhaustion, how much energy can be pulled from them continuously, charging procedures, safe discharge and storage procedures, etc. Here are some particulars for each.
LiPo
350-800 charge /discharge cycles
Can age quickly when abused
Must be balance charged
Only 3 cycles needed for break
Per cell:
Nominal voltage 3.6-3.7v
Safe operating range 4.2-3.3v (4.2 FULL charge)
3.3v cutoff
Safe low value 3.3-3.5v
LiFePo4
2K-5K cycles or more (up to 8K for some)
Must be balance charged
Handles more abuse and aggressive charging than LiPos
Stores well
20 cycle break in
Per cell:
Nominal voltage 3.2-3.3v
Safe operating range 3.6-2.6v
2.8v cutoff for longer life
Safe low value 2.6v
10 year or longer life
Resists heating up when used heavily
NiMH
200 - 2K cycles (Fast charging can wreck a battery within 250-300 cycles.)
No balance charging required
Per cell:
Nominal voltage 1.2v-.9v (Can start as high as 1.5, but drop to 1.2 almost immediately.)
Safe operating range .9v-1.2v. -Damaged at .8v
By comparing some common offerings, we can get an idea of the available energy of differing packages. Watt Hours, with a minimum voltage level suited to the onboard electronics, is a good measure of suitability.
NiMH Energy
8.4v 4200mah 35.28wh
8.4v 5000mah 42wh
7.2v 3000mah 21.6wh
7.2v 4000mah 28.8wh
(7.2v=6 cell, 8.4v=7 cell)
LiFePo4 Energy
9.9v 2100mah 20.79wh
9.9v 3000mah 29.70wh
9.9v 4200mah 41.58wh
(3S, 3 cells in series)
LiPo Energy
7.4v 1500mah 11.1wh
7.4v 4000mah 29.6wh
7.4v 5200mah 38.48wh
11.1v 2200mah 24.42wh
11.1v 4200mah 46.62wh
(7.4v=2S, 11.1v=3S, 2 and 3 cells in series)
It is obvious why so many go with LiPo batteries. For a given pack size, the energy density is higher. Some truly impressive run times are possible. When I did work ups of power needs on a few machines, I settled on something in the neighborhood of 30wh for a typical equipment load, and a voltage above 8v as a safe standard. 7.2 or 7.4v is too close to the minimum 7v required by the most VTxs in use.
The first NiMH in the list is 35wh. It does the job. But it is big and heavy. The second LiFe unit does about 30wh, and satisfies my power needs. It is also about 35% lighter.
“Why not go with the LiPo? It does EVERYTHING.”
Well, I’ll tell you. The operating environment for inspection rovers involves close proximity to a lot of dry timber. If a fire were to start in that confined space, reaching it and putting it out would be very difficult. LiPos are known for dramatic combustion under certain failure conditions. When a LiPo lets loose, it often becomes a violent flaming torch, producing huge volumes of thick, acrid smoke. It can make a room uninhabitable in 15 seconds or so. If this happens in a crawlspace, no one is getting to it without some serious saws. Restricted movement, zero visibility, toxic air and flames set the rules. The fire will burn less than two minutes, typically - but long enough to ignite the crawler and the nearby wood.
LiPos experience unsafe changes in chemistry when they are repeatedly over-charged, discharged, over-heated, or stored at too high a charge level for extended lengths of time. Additionally, impacts, cuts, punctures, water or oxygen invasion, can all ignite a recently healthy battery pack. The liability and injury exposure is too great, in my opinion. Explosive torch events are rare, but not so rare that you can’t find examples of them on YouTube. For these reasons, I’ll never run a LiPo on an inspection rover.
“Aren’t LiFe batteries also Lithium batteries?” Yes, they are. Sort of …. They are Lithium Iron Phosphate. Their chemistry is significantly different from Lithium Polymer, or Lithium Ion batteries. LiFe (LiFePO4) is very forgiving of abuse, tolerant of destructive environments and rather tame when they fail, compared to LiPos. At most, you could expect large volumes of smoke. LiFe batteries do not fail with dramatic flames. Several comparison test videos of all three battery types show that NiMH and Life chemistries are tolerant of axe hits, nail guns, torches, over /undercharge, impacts…. while LiPos go full demon in most cases.
“Can I switch my LiPo out for one of the others?”
Maybe. Considerations are space, weight, suitable voltage, battery charger, ESC configuration and connections. It would be best to consult a builder for advice.
Dewalt LiPo Equipped Rovers
Some commercial rovers use in modified Dewalt battery packs. Are they safe? I can not remember ever hearing about one going nuclear. Dewalt chargers and batteries use a pretty good BMS (battery management system). A BMS monitors the critical items related to charge and discharge, and protect the pack from abuse. The issues I see with these are size and weight. If your rig can handle both, I see no immediate reason to reject them. To date, they are only mounted on tracked vehicles, and behind the midpoint, where weight could present balance problems when crossing obstacles.