Batteries:
When designing a robot, batteries are often one of the most critical component decisions you can make. My partner and I at Inertia Labs have tried just about everything out there, and below you will find the most useful bits we have figured out.

Battery usage in robotics falls into two categories. Low voltage/amperage batteries for powering radios, receivers and other low draw devices. And high voltage/amperage batteries used in drive and other high power applications. (It is also possible to combine the two uses into a single battery solution with a voltage regulator or 'battery eliminator'.)

note: If you are confused by any of the terms in this primer see the definitions at the end of this page. If you are just starting in electronics the How Stuff Works website has sections on batteries, and general electronics that are helpful. Also buying a remote control car kit and assembling and running it will teach a lot of the basics of robot building.

You can buy or assemble batteries in just about any size, weight, voltage, capacity and amp outputs. However as with any design problem, you must choose which of these key attributes you want, you don’t get them all. Generally for batteries in combat robots the most critical trade-off is between sustained amp output and weight. I differentiate amp output vs capacity, or amp hours here, as amp hour capacity ratings turn out to be very misleading in high draw robotic applications.

Your electric motor's horsepower or wattage equals your battery voltage times the amps your motor draws. If your battery set cannot put out enough sustained amps, or the voltage sags too heavily under load, you will not get the maximum power your motors can deliver. As you draw power from a battery the voltage will begin to sag, a 24v battery can easily sag all the way down to11v under heavy load. The more amps you draw the more voltage sagging will occur, and the less efficient your motor becomes. Voltage sag is something few people take into account regarding their batteries, but it is just as important as amp output in your horsepower equation.
There are many different battery chemistries available but for the purposes of this article I will only discuss the most useful and available types as they pertain to robotics.

Sealed Lead Acid (SLA):
These are batteries that have been developed for the communications, sport vehicle, and electric vehicle markets from the age old lead acid battery technology. These batteries can surge a ton of amps (often up to 1200), come in nice 12v packages and are often the heaviest battery solution. While they can peak the most amps for a split second, they cannot sustain high amp draw very well. It is also possible to make your own SLA packs out of 2v lead acid cells. This allows more versatility in battery configuration and voltage if needed. You will want a charger that is specifically designed for SLA batteries.

Nickel Cadmium (NiCad):
NiCad batteries are some of the most commonly used rechargeable batteries in electronic devices. Because of this there are many sizes, grades, and application specific NiCad solutions. These tend to come in 1.2v cells that have to be assembled in series into packs of a usable voltage. Next to SLA, NiCads have the second highest amperage output, and are far better at sustaining high output than SLAs. The most commonly used size in robots is the sub C cell size and the full C cell size. The good ones range in capacity from 2.4Ah to 3.6Ah and can put out bursts of up to 200 amps and hold 80 amps of output for a couple minutes. To charge them you will need a charger designed for NiCads and the number of cells in your pack.

Nickel Metal Hydride (NiMh):
NiMh batteries are now often seen in portable phones and computers and as loose rechargeable cells. They come in the same cell sizes as NiCads but are generally lighter and cannot put out as many amps. These are also easier on the environment when it comes time for disposal. The chargers are usually identicle to the NiCad chargers but you often charge them at a slightly slower rate.

Lithium Polymer (LiPo):
LiPo or LiPoly batteries are one of the more recent and promising battery chemistries. They are generally made in 3.2v flat rectangular cells. These are most often used in low draw applications, but the more recent versions can sustain fairly high draws. We are using them in 1lb combat robots now and have been very happy with them. LiPo batteries are easy to package because of their rectangular form, and are the lightest power weight ratio you will find readily. They require a specific charger made for LiPoly and the number of cells in the pack. It should be noted as well that they can have a dangerous failure mode in high draw or fast charging. They have been known to catch fire in some circumstances. This seems to be getting much better as the chemistry, packaging and charging technology evolves.

small 12v packs made with AA and AAA cells

Low Draw Applications: (under 12v, and under 2 amps)
For receivers, radios and other low draw applications it is possible to use very light batteries. To power our 12v IFI radio which draws less than one amp at 12v we use a series of 10 "AAA" NiMh cells to form a pack that will power our radio for an hour or more of constant on time and weighs only a few ounces. You can also get these packs in 4 cells to power standard RC type receivers. This is also a great place to use LiPo batteries.

It is also possible to use a voltage regulator on your main battery system to power your radio and other low power items, but we have found some disadvantages to it. When super high loads go on your main batteries the voltage drops heavily. Depending on how severe this is, it can cause your radio to shut off intermittently as these large voltage dips occur. Since it can be done for a low cost in weight, we have found a separate pack is often a better route. (It is also worth mentioning that you could use disposable alkaline or lithium batteries for these types of applications which may last the longest but its expensive, wasteful, and wont save any weight.)

20 cell 24v pack made with C cells

High Draw Applications: (12-48v and 50 to 1000+ amps)
These are your main batteries. The voltage and amperage you need will depend on your motors, gear reductions and the ability of external forces to stall the motors. (Applications above 48v DC are not covered in this document, if you plan on using voltages that high you should be aware of the dangers, and already be an expert in this stuff.)


One of the most common confusions about batteries in robotics is around the Amp Hour ratings most manufacturers use to rate the capacity of their batteries. The problem is that this rating is heavily dependant on how many amps you are trying to get from the battery and the battery type. For instance an SLA battery system rated for 18 Ah will only put out a tiny fraction of that when being loaded at several hundred amps for a few minutes. It is often very difficult to get high draw ratings on batteries for 3 to 5 minute total run times, and so we are often left to experimentation. Batteries also tend to get very hot when used this way. We use temperature "tell tale" stickers that tell us how hot batteries and other parts of the robot get during practice and competitions which are very helpful.


Battery outputs and charge rates are often rated in a “C” rating. 1C means that its output rating is equal to its amp hour rating. So an 18Ah battery rated at 1C output is rated for an 18 amp draw. These ratings are often very conservative and are listed as continuous or peak. Peak usually means for a split second not a minute or two.
What is critical to robot builders is finding a battery that can put out the surges of power needed to get a powerful DC motor running and then sustain a portion of that for the time needed in your application with continual spikes of demand for turning, driving and pushing. In the graph below you can see what kinds of demands might be put on a set of batteries in a modest 24v drive system during the first ten seconds of a combat robotics bout. This bot draws about 55 amps nominally when moving but since it has to overcome a lot of tire friction when it turns (due to tank style turning) or pushes against something, it has peak demands well above 200 amps. You can also see that the demand peaks when the motors start moving from a stand still.

The usage curve for weapon motors is also similar as it takes a certain amount of power to get a spinning disk up to speed, but once its spinning it uses a nominal amount of current, which is interrupted by spikes of demand every time the weapon meets resistance (like the other robot).

So how do you decide which batteries are for you? As you can see there are a lot of factors. Since it is impossible to understand every situation I will go through our battery decisions with our combat robot Toro and hopefully you can apply the same logic to your robot and just scale the numbers appropriately to make your choice. In Toro, a 340 pound super heavy weight combat robot, we use four 24v motors that draw 180 amps at near stall. (Most places selling motors for robots like NPC, can give you the stall current for your motors. Otherwise you can use an ammeter to find your motor draw.) Our total potential draw could be over 700 amps in our drive train if all the motors stalled (not very likely). We geared these 3000 rpm motors at about 7 to 1 and used wheels of about 11inch diameter. This gives Toro a top speed of about 13 m.p.h. while still having enough torque to spin the wheels when pushing against something. (This slippage is important as it means our motors rarely draw their full stall current which helps our battery life and speed controllers.)

We originally put two 12v Lifeline SLA 18Ah batteries in series to get 24v. These weigh about 31 pounds combined. The Lifelines gave us really good power for the first minute or so. (They could surge up to 1200 amps) but after 2 minutes the robot was very sluggish and at 3-5 minutes of hard driving the robot barely moved. We then went to two 12v Hawker Genesis 16Ah batteries because they were a few pounds lighter and we had heard such good things about them from other builders. These turned out to give us even less run time than the Lifelines (With nearly the same chemistry, the 2 less amp hours really did matter in this case.) Then we put in six 3Ah NiCad BattlePacks in parallel which weighed only 24 pounds. The additive amp hour rating was again 18 Ah, but the NiCad battery chemistry, and the parallel configuration work in our favor to sustain current for much longer. Toro now has over 10 minutes of run time while still saving 5-7 pounds and taking up less space than SLA batteries.

Intercooling:
One of the latest developments in high draw NiCad and NiMh applications is intercooling. Inertia Labs has pioneered new designs of high heat materials and intercooling fans which drastically increases the amount of power that can be safely pulled from a battery pack in the 3 minutes of combat. Robotic Power Solutions will have these new designs available this 2002 season.


Series

The diagram above shows 1.2 volt cells in series, notice how the cells are connected positive to negative to make their voltages addative (6v total). This battery pack, if built with 3Ah cells which can put out 80 amps each, would result in a battery of the same Ah rating and amp output rating. So in series construction voltage is addative but not Ah or total amp output.

 

Parallel

The diagram above shows 1.2 volt cells in parallel, notice how the cells are connected positive to positve and negative to negative which makes their voltages stay the same(1.2v total). This battery pack, if built with 3Ah cells which can put out 80 amps each, would result in a battery of 18Ah rating and 480amp output rating. So in parallel construction voltage stays the same but Ah and total amp output is additive.


Whole battery packs can be put in series or parallel as well having the same additive effects to voltage or amp output. Some battery pack makers also offer 'cell matching' and 'cell zapping' or 'voltage enhancing'. It is our experience that cell matching does not make a huge difference since we are putting many packs in parallel and to match all the cells and the packs would be near impossible. Also we dont use the cells to the end of the packs life where matching becomes critical. Zapping or Voltage Enhancing however does seem to decrease internal resistance in cells making them able to put out more amps slightly longer.

Charging and Care:

Charging for each type of battery is a bit different. One of the best ways to ensure good care for your batteries is with a log. many chargers tell you how many amp hours and what the peak voltage was when charged. By keeping track of this and other factors like run time, and total number of charge cycles you will quickly see how to get the best performance out of your batteries.

SLA batteries need to be charged with specific chargers made for their chemistry. Not all SLA batteries are the same. Some use Absobed Glass Mat (AGM) and or Spiral cells which require special charging. Most good charger manufacturers like ChargeTek and Guest have chargers made specifically for these high perfomance SLA batteries and you should talk to them about your batteries when ordering. SLA batteries should generally be stored with a full charge or with a battery maintainer on them as if left uncharged for extended periods they will be ruined.

NiCad and NiMh batteries generally use 'peak charging'. This means that you charge them with increasing voltage until it peaks and then stop. Most good chargers take care of all this for you. You will need a charger that can hanle the number of cells in your packs. Most hobby chargers are only designed for 6-9 cells making a 20 cell 24v pack out of its range. Most used chargers in combat robotics for NiCad and NiMh are the Shultz, Graupner and Astroflight which can charge 30-40 cell packs. There are several other manufacturers currently developing chargers for more cells too. Periodically it is wise to fully discharge these packs to minimise 'memory' effects. These batteries should generally be stored in a mostly discharged state. If your batteries are getting hot while charging it is a good idea to use a fan to keep them cooler during charging.

LiPo Batteries require a specific charger for that chemistry and the number of cells. Some high end hobby chargers like the Schultze will charge LiPo as well as NiCad and NiMh making them a good buy. There are also many cheap and good LiPoly chargers for up to three cell packs sold for the hobby industry.

Inertia Labs Battery Reccomendations:
All batteries are not created equal. Below are some recommendations on brands. It should be noted that whichever type of battery you use there are very few that will work in a combat robot due to high perfomance requirements. If you dont use any of the ones listed below be sure you talk to your vendor extensively about the application before making a purchase.

Terms:

by Alexander Rose (c) 02005

 

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