You’ve built the model. Now how do you power it? This Basic Topic helps you select the right battery and offers guidance on how to operate it for maximum performance and long life.
So far all Armortek models except for the Universal Carrier have required a 24V power source.
The factory prototypes run on two Yuasa REC 12-22 12V sealed lead acid (SLA or VRLA) batteries in series, capable of 22Ah, which will typically give an hour or more of running time. Endurance will depend on ambient temperature, ground conditions, weight of model and the degree to which the running gear has been assembled true and free. All batteries wear out after a number of charge/discharge cycles. You can prolong useful life and maximise performance by correct operation and maintenance.
Batteries age prematurely when not properly maintained and when operating limits are exceeded. Batteries deliver reduced voltage when under load (ie supplying power). This effect is the result of internal resistance in the cell(s). This “voltage drop” varies with the amount of current flowing. As you draw more power, the voltage drop will increase. As a battery ages, the internal resistance increases and the battery’s useable voltage will be reduced. You’ll notice this in reduced performance of the model – it will accelerate slower and struggle going uphill.
For all models including the Universal Carrier, Lithium Iron Phosphate (LiFePO4) batteries are a modern, efficient, alternative (See below).
Sealed Lead Acid or Valve Regulate Lead Acid (SLA or VRLA) Batteries
Absorbent Glass Mat (AGM) SLA batteries are maintenance free and can be mounted in any orientation. Gel Cell SLA batteries are an earlier type which have similar qualities but are more suited to lower discharge rates than are seen in our models. They should be restrained in the hull, using straps. Some builders install a bespoke battery tray, using aluminium angle.
How long will the power last?
You will only be able to use about 50% of a SLA battery’s capacity. Have a look at the discharge curve below to see why. We need to understand the term C rate: amount of current flowing through a battery as a multiple of the battery capacity. For example, the output current of a 22Ah battery discharging at C/1 (or 1C) would be 22 amp. At C/20 (0.05C) would be 1.1 amp. Normal charging rate for a Pb battery is C/10 (0.1C), so for a 22Ah battery, it would be 2.2 amps. Now for the graph below.
The graph shows depth of discharge against battery voltage. The curves show how long it will take to discharge a 12V SLA battery to the point where it’s not delivering usable power. The time is dependent on the current drawn. The red box shows that a constant discharge current equivalent to the battery capacity will run out of steam after an hour (eg 20A for a 20Ah battery). At 1/20th capacity current, the time extends to about 20 hours. In reality, our models will draw a very variable current, depending on how the model is driven, the ground surface, the temperature and the age of the battery.
For the typical 22Ah array found in our models, expect about one hour of running. Although we don’t have comprehensive data for power consumption for our models, peak current draw of up to 60A has been seen. In practice, with SLA batteries, you will notice a gradual fall-off of speed and pulling power. If you run for too long, you may well find you have no more useable power and recovery of the model is then a real challenge.
Charging
How do I know if the battery needs charging? Fortunately, for SLA batteries capacity is proportional to voltage, so measuring voltage will give you a good indication of power remaining. The latest Armortek motion packs have a battery indicator on the Power Module, which will give a rough indication of remaining capacity (for SLA batteries only). To supplement this, a multimeter can be used to measure voltage. The ideal answer is to use the telemetry capability of many modern radios, such as the Spektrum and Fr Sky ranges. With a battery voltage sensor fitted in the battery harness, you will be able to see battery voltage under load in real time. Note that the on-load voltage will be lower than resting voltage ie there will be some measure of recovery when the model is at rest.
The nominal cell voltage of a SLA battery is 2.0V. At a resting voltage of 12V, a six cell SLA battery is in fact only charged to about 15% of capacity. When fully charged, the resting cell voltage will be about 2.11V ie 12.66V for a 12V battery. Note that this is the resting voltage, which will be lower than the voltage when charging (see below). If you want to measure resting voltage after charging, leave the battery for at least 30mins to allow voltages to stabilise.
SLA batteries are more tolerant of overcharging than other types but repeated overcharging will shorten life . They also self-discharge. It will not harm SLA batteries to leave them fully charged but expect to do a top up charge if left for a long period. SLA batteries require a constant voltage charge regime but in order to achieve the best balance between shorter charging time and longer life, charging in three stages is ideal:
- Bulk Charging: replaces 70-80% of capacity at a relatively high and constant current, until the battery reaches an on-charge voltage of 14.40-14.70V.
- Absorption Charging: takes the battery to about 98% capacity, with a reducing charge voltage, until the battery has an on-charge voltage of 14.10-14.40V
- Float Charging: holds the battery indefinitely at a sustainable voltage of 13.5V by balancing the charging voltage with the self-discharge rate.
How do I achieve this ideal profile? The answer is a so-called intelligent or smart charger, which adjusts the charging voltage to match the profile above.
Although some chargers offer 12V and 24V options, charging both batteries in series is risky unless both batteries are in the same state of charge. It is better to charge each of a series pair with a dedicated charger. SLA batteries have failed early because they've been charged in series and one has been over-charged as a result. It's better for the batteries if they can do their own thing when charging.
If you decide to use SLA batteries, be prepared to keep your model connected to mains electricity when not in use or accept reduced battery life. With SLA batteries, the model will lose performance as the battery is used and the batteries will not deliver effective power after about 50% discharge.
If left in a discharged state, an SLA battery will be damaged and will not be able to be recharged
Lithium Iron Phosphate (LiFePO4)
LiFePO4 batteries are a good alternative to SLA. They are more expensive but last longer. They deliver considerably more useable power than SLA. They are lighter and smaller.
Over the life of your model, LiFePO4 batteries will cost less than half the cost of SLA per cycle. This is a worst-case assumption for two reasons. The figures assume a best-case life for SLA of 500 cycles. In practice, our driving profile will reduce this life considerably. Secondly, the quoted price of LiFePO4 is for a high-end battery (Tracer) with charger. Prices are coming down and it is now possible to find a 24V 16Ah LiFePO4 battery for as little as £324 or two 12V 20 Ah for £270. These would have a whole life cost of £0.16/ cycle and £0.13/cycle respectively. If you want to see this graphically, the difference is pretty stark.
Money isn’t everything, there is one overriding advantage to choosing LiFePO4 and that is about getting more useable power. Have a look at the discharge curve for LiFePO4, compared with SLA:
The LiFePO4 curve is much flatter. That means you won’t see the battery start to flag – it will continue to deliver nearly full power until it is close to exhaustion. Beware though, that at that point, the curve falls off a cliff and the model will come to a grinding halt. In practice, you’re more likely to get bored before you run out of power. That point is easily avoided by monitoring the battery condition as with SLA. Most LiFePO4 batteries are supplied with a battery condition indicator, battery management system and charger.
The flatter curve gives you one more advantage – you can use a battery with lower capacity compared with SLA and get as much or more useable power. If a typical SLA array requires 22 Ah, with LiFePO4, the equivalent would be around 16Ah. In the comparison chart above, this would also reduce the weight, size and cost.
One word of warning. It's important to check the specs of any LiFePO4 battery before using it in an Armortek model, to be sure the max discharge rate won't be exceeded. Some of the cheaper, smaller batteries have very low max discharge rates. You're looking for somewhere around 60A momentary. The Tracer pack above has a max continuous discharge of 30A and a peak of 60A for 10mS. There was a concern that the Armortek power regeneration system might over-charge a LiFePO4 battery but experience has shown that this is not the case.
It’s also important that the LiFePO4 battery comes with a Battery Management System (BMS). This protects the battery against over charge, over temperature, over current and deep discharge. In effect, it gives you care-free handling. If you do exceed any of these parameters, the BMS will cut off power. It’s easily restored after a short wait by connecting a charging voltage briefly. Most LiFePO4 batteries come with a dedicated charger, which also protects against over-charge. If you carry this and a small 24V battery, you will always be able to reset the BMS in the field.
Which Type of Battery?
So, SLA or LiFePO4? It really comes down to a personal choice.
SLAs have been the default for Armortek models and over the years have provided a good solution for most people. They are relatively cheap and commonplace across the globe and can manage high charge and discharge rates safely.
LiFePO4 on the other hand, whilst more expensive and needing to be paired with a battery management system (BMS) are becoming more and more commonplace as the prices come down. The two main advantages of LiFePO4 over SLA are that (a) they are much more compact and lighter than SLA, allowing more flexibility in placement of the batteries, and (b) they provide a significantly longer usable power duration in the field.
Universal Carrier
The Universal Carrier is a different case all together. It requires 7.4V and the battery compartment is 200 x 60 x 40mm. These constraints require a different solution.
The factory prototype uses a 7.4V (Two Cell – 2S) Lithium Polymer (LiPo) battery with a maximum discharge rate of 40C.
Safety
LiPo batteries are widely used in everything from mobile phones and laptops to drones, RC cars and the like. In general, they are safe but they must be handled correctly, operated within limits and importantly, a proper balanced charger needs to be used. If mishandled, LiPo can suffer a thermal runaway, resulting in an intense and toxic fire, which could destroy the model and cause injury and loss. In risk terms, the chance of occurrence is low but the impact is high.
LiPo must be removed from the model when not in use and particularly when charging. The battery on the UC is mounted externally and is easily removable. On all other Armortek models, the battery is mounted internally and LiPo are not recommended. Regenerative power currents on the other models may also exceed LiPo limits and create an extra risk.
7.4V LiFePO4 batteries are also available and might be worth considering.
Know your battery - Terminology – what do the numbers mean?
The capacity can be expressed in Ah or mAh (2.2 and 2200 above). On some batteries, you may see something like 2S1P or 2S2P. The S refers to cells in series, the P to cells in parallel. 2S1P is a battery with two cells in series to double the voltage. A 2S2P battery has a four cells, two pairs of cells in series, connected in parallel, to double the capacity.
To operate a LiPo battery safely, you need to know some critical voltages. The nominal voltage of a LiPo cell is 3.7V. That’s why a two cell LiPo is quoted with a nominal voltage of 7.4V. Take a look at the following graph:
We can safely use cell voltages between fully charged at 4.2V and 50% discharged at 3.85V. When any cell reaches 3.85V, we should really stop running. This will leave a margin, down to 3.75V where we can operate with care, for example in loading the model into a vehicle. At 3.5-.3.6V, the cell has reached a critical level. Below this, there will be insufficient useable power and a considerably increased risk of a thermal runaway ie a serious fire. Running on below 3.5V per cell will cause puffing and significantly reduce the battery life.
We can see these figures for a two-cell battery on the following chart:
How can we monitor battery voltage?
To operate a LiPo battery safely, we need to monitor the battery in real time to avoid getting into a state where the risk of fire increases. It’s vital to know how each cell is performing and a battery monitoring system is a very wise investment. This should include a battery checker which allows individual cell voltages to be read when the model is static. This monitor is simply plugged into the balance lead and reads individual cell voltages.
The ideal is a telemetry system giving a readout of voltage in real time. Fitting the sensor is simple and cheap. For Spektrum and Futaba RC systems, power pack total voltage is presented, together with maximum and minimum values.
With the Fr Sky systems such as Taranis X9D, in addition to total battery voltage, readouts include individual cell voltages and the minimum cell value.
Charging
LiPos must never be over-charged. Any cell charged beyond 4.20V is a fire waiting to happen. Above 4.25V, LiPo cells will start to vent oxygen, which will cause the cell to puff up and ultimately spontaneously combust. A short excursion beyond 4.20V probably won’t cause permanent damage.
LiPos also require balanced charging. This means that each cell is held in a very similar charge state to its neighbours. Charging without a balancer can over-charge or discharge an individual cell to the point of danger. Fortunately, battery chargers with a dedicated LiPo mode, including balance charging, are widely available and are essential for safe operation. Connect the battery power lead to the charger battery output lead, the battery balance lead to the appropriate charger socket for the number of cells and select balance charging.
You will also need to select the charging current. Although some battery/charger combinations support fast charging, it’s safe to charge at 1C ie a current in amps equal to the capacity of the battery. For example, a 2.2Ah battery should be charged at 2A. Easy really. The charger will have a cut-off which protects against over-charging. Most chargers will also give a readout of individual cell voltage and internal resistance and it’s a good idea to check these at the end of charging.
The increased risk of fire during charging can be mitigated with some sensible precautions:
- You must use a LiPo battery charger
- Remove the battery from the model, and charge in a fireproof container. LiPo safe bags are available but perhaps the best solution is an ex-military ammunition box. If using the latter, drill a couple of holes in the side to vent any gases in the event of fire.
- Don’t leave a battery under charge unattended
Storing
With other battery types, we’ve been used to leaving fully charged batteries in our models for long periods. LiPos require a different approach. Most balance chargers have a storage setting which will charge/discharge to the optimum voltage of 3.80V.
LiPos have a very low self-discharge rate and don’t require top-up after fully charging.
LiPos should be stored in a fire-safe container. Coloured end caps for the mains lead are a good idea, green for fully charged and red for storage. They will also protect against short circuits.
How will I know when it’s time to discard a lipo?
Mistreat a LiPo and you’ll be lucky to get more than a few dozen cycles out of it. Follow the guidelines and it should be good for 200-250 cycles. Recording your cycles will give you a feel for battery ageing. If at any time, the battery starts to swell, it’s a sign of internal gassing. That will cause the internal resistance to rise and power to reduce. If swelling is significant, discard the battery. Internal cell resistance increases as a LiPo ages. Most intelligent chargers allow the cell resistance to be read. A typical value for a new battery would be around 4-5 ohm. Once you see it rise to 10-12 ohm, it’s time to think of discarding the battery. LiPos should be fully discharged before disposal. There is plenty of guidance online for this procedure.
If you want your LiPo to last, follow these 3 rules-of-thumb:
- Don't discharge lower than 20% remaining capacity ie 3.73V/cell
- Stop the recharge at 95% of capacity ie 4.15V/cell
- Store packs at 3.80V/cell.
Further reading:
http://www.gibbsguides.com - Rather technical guides for SLA and LiPo batteries. Payware
https://www.promodeler.com/askJohn/How- ... LiPo-Packs - good clear guide to Lipo care
http://www.bvrmc.uk/pdfs/lipo%20safety.pdf - British Model Flying Association Safety Guide for LiPo
https://rogershobbycenter.com/lipoguide/ - Another straightforward guide to LiPo
https://batteryuniversity.com/ - Comprehensive online resource for batteries in general