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Lithium Polymer batteries for ham radio applications

This article documents a preliminary exploration of the application of Lithium polymer batteries to ham radio.

The exploration is of two Lithium Iron (LiFe, LFP or LiFePO4) batteries, which though a little harder to get than the more common types of Lithium polymer batteries have some advantages for ham applications (see the Wikipedia links at the bottom of this article), particularly safety and slightly lower cell voltage that better suits common nominal 12/13.8V systems.

Fig 1: 
 

Fig 1 shows a 3 cell LiFe 1500mAh battery of nominally 9.9V, and rated at 1C (1.5A in this case) maximum charge and discharge current. The battery is 103mm in length and weighs in at 124g including cables and connectors, so has a capacity of 430kJ/kg (contrasting around 120kJ/kg for a 12V 7Ah SLA battery). 8.50=8.50/=$0.16/kJ

It is a low current battery intended for low drain applications such as power electronics and is ideally suited to powering accessories that would have been powered from a 9V power supply or battery. The downside of these cells is internal leakage, so they need to be periodically charged even if only used lightly.

As can be seen, the original connectors were cut off and both red/black pairs terminated in an Anderson Power Pole connector. Note, this is not fused at this point, and adapter cables connecting equipment need fusing appropriately. The connector on the top side of the picture is a 4way JST PH series connector for a cell balancing charger.

Fig 2: 

Fig 3 shows the warning label on the back of the battery. 

Warning: do not change the connectors unless you are competent to work on a live un-fused battery, if a short circuit is created, serious damage to persons and/or equipment may occur.

Fig 3: 
 

Fig 3 shows 4 cell LiFe 4200mAh battery of nominally 13.2V, and rated at 30C (126A in this case) maximum discharge and 1C (4.2A in this case) max charge current. The battery is 140mm in length and weighs in at 520g including cables and connectors, so has a capacity of 380kJ/kg (contrasting around 120kJ/kg for a 12V 7Ah SLA battery). 54.50 $/kJ0.27

It is a high current battery intended for high current applications such as traction in RC models and is ideally suited to powering low power transceivers that would have been powered from other 12V battery technologies or a power supply. The higher energy/mass would be a real benefit if the equipment was being carried any distance, especially up mountains!

As can be seen, the original connectors were cut off and red/black pair terminated in an Anderson Power Pole connector. The 10mm2 (~#7) conductors are a very difficult fit in 45A Power Pole pins, but with care they can be crimped and a very sparing application of solder to help prevent the working out of the crimp. Note, this is not fused at this point, and adapter cables connecting equipment need fusing appropriately. There is also a 5way JST PH series connector for a cell balancing charger.

It is worth noting that the battery has a clear warning [p]lease read safety warnings and usage guidelines at www.zippybattery.com before use, the website does not appear to exist. Though the seller lists specifications that include a maximum charge rate of 2C, there is a clear warning on the battery to not exceed 1C charge rate.

Warning: do not change the connectors unless you are competent to work on a live un-fused battery, if a short circuit is created, serious damage to persons and/or equipment may occur.

Fig 4: 

Fig 4 shows the warning label on the back of the battery.

These cells should not be completely flattened, they are likely to be damaged if cell voltage falls below 2.8V, so a battery monitor is advised.

The outer covering of the batteries is fairly easy to damage, something cutting or piercing the covering writes the battery off and may well cause a fire. The battery can be toughened by a layer of clear heat shrink, 50mm dia heat shrink suits the 3 and 4Ah 3S batteries in use.

Charger

Most of the chargers shown here create substantial radio noise on HF.

Fig 5: 
 

Fig 3 shows the IMAX6 charger. At first inspection, this looked like a well made charger and good value at around A$40 without power supply. That assessment changed when it was time to connect batteries as the end plate interfered with the upper row of JST PH sockets and they could not be used. Careful nibbling of 2mm from the upper side of the aperture allowed the plate to be refitted with adequate clearance for the sockets and they could be used.

The 4mm banana plugs at the right are the battery connection and in the supplied adapters, un-insulated male parts are live and a risk of short circuit of an un-fused battery. Fusing these charging adapters mitigates this risk somewhat, but it is a design failure to put live battery on exposed male connectors.

The charger is available with a 5A switch mode power supply, but in my case, I chose to buy a power supply from a known source for reduced risk of troublesome RFI, and the cost was similar.

Fig 5a: 

Fig 5a shows a B6 charger with DC or mains input. 

Fig 5b: 

Fig 5b shows the Accucell 8150, a higher powered charger. 

Fig 5c: 
 

Fig 5c shows a switched mode 14V 25A power supply for powering the chargers. Note that his is not a power supply suitable for directly powering a nominal 13.6V radio.

 Four of the Accucel 8150 chargers on this power supply can charge four 3S or 4S 4Ah batteries in a little over half an hour.

Connection accessories

To mitigate risk of short circuit and subsequent adverse outcomes, the batteries are used with fused accessories for powering loads.

Fig 6: 
 

Fig 6 shows two examples, the gray leads are 2.1mm DC plugs on a fused lead for connection to the battery using Anderson Power Poles. The other device is an ATC fuse holder in a short section of conductors with Anderson Power Poles for insertion of a fuse between the un-fused battery and un-fused adapter leads or loads.

The RC world is awash with adapters for parallel charging these batteries. This is to my mind very short sighted and very dangerous. Batteries with low internal resistance should NEVER be paralleled unless their is zero difference in their terminal voltage, otherwise a large current may flow from one battery to the other to equalise the charge state. LIPO batteries can supply large equalising currents that may exceed safe operating conditions and result in damage, explosion or fire. Buy another charger, or one designed to independently manage several batteries.

Other accessories

Inline Wattmeter

Fig 7: 

Fig 7 shows an inline Wattmeter which is quite a neat combination of facilities. Important shortcomings are:

Fig 8: 
 

Fig 8 shows another inline Wattmeter, this one with facility to monitor individual cell voltages. The HobbyKink HK010 also senses current in the negative lead.

 Low voltage alarms

There are a range of low voltage alarms which give an audible and visual alert when battery voltage drops below an equivalent of 3.3V per cell (which could damage the cells). Some also display the battery voltage.

Fig 9: 
 

Fig 9 shows a type of monitor available. This cycles through displaying each cell's voltage, then overall voltage and has an adjustable cell alarm point and loud alarm for low voltage an any cell. There is a trap with this monitor, is powers itself from the 2S point, so it unbalances the battery over a period of time, easily observed with the instrument itself. They are for checking rather than continuous monitoring. In selecting a monitor, ensure that it is configurable for your cell type.

Fig 10: 
TURNIGY 3~8S Voltage Detector

Fig 10 shows a type of monitor that connects only to the full battery, so does not unbalance the battery in use. It is configured for LiPo battery voltages which are different to LiFe.

Fig 11: 
 

Fig 11 shows a battery checker that can be configured for different cell types.

Paralleling cells

The RC market uses some quite unsafe practice in batteries and related accessories. Adapters are readily available for paralleling batteries or cells for increased capacity, but many appear unsafe.

If two cells at different terminal voltage (eg due to different states of charge, temperature etc) are paralleled, a current flows to equalise the internal voltage of each cell. Although the voltage difference might be quite small, the internal resistance of Lithium batteries is extremely low and dangerously high current may flow for a short period while the cells equalise.

The same thing applies to paralleling batteries.

Adapters that allow batteries to be paralleled are potentially dangerous if the batteries are not equalised in voltage BEFORE the adapter is connected to the batteries. A problem of these batteries when paralleled is that the facility of 'balance charging' is lost as there are now two balance connectors, one for each battery and even though the batteries may have equal voltages by vitrue of an adapter paralleling the main discharge terminals, the cells are not necessarily all equal from batter to battery.

Never mind, someone with little knowledge of batteries thought a a solution, another Y adapter to parallel the balance connectors. For the reasons discussed above, this is potentially dangerous whenever the two balance connectors are first connected (eg if done so each time a battery pack is balanced charged).

The above issues apply less to a battery pack that has internal parallel links (eg 3S2P battery) as the batteries are supplied with the battery and corresponding cells matched by virtue of the internal links, and it can be used just like a non-paralleled battery. It is ad-hoc external parallel links that creates the issue EVERY time they are connected.

Concerns

Transport and storage

Popular discussion has copious warnings of risk of fire from Lithium Polymer batteries.

LiFe batteries are safer overall than other types of Lithium Polymer batteries

Short circuit of a battery or cells risks explosion and fire.

Part of the problem stems from the fact that batteries such as the above are pouch cells, they are not in a rigid enclosure and so the risk of puncture of the cells is greater. To mitigate this risk, batteries should be adequately protected from risk of puncture.

Part of the problem may be associated with the types of connectors and connections. The use of connectors that are un-insulated or part un-insulated (such as alligator clips) raises the risk of short circuit, and that occurring at point with no fault current limiter (eg fuse) could be catastrophic. To mitigate this risk, use properly insulated connectors and fuse connections to the battery.

The batteries are sold to the RC market with a wide range of connectors, some of which risk short circuit of the un-fused supply if they connect metal (eg the so-called T connector with exposed live male pins). Use connectors that minimise the risk of short circuit of live parts.

With some types of connectors, it may be easy to short circuit the power supply briefly when coupling the connectors (eg common 2.1mm DC connector, TS 3.5mm plugs). When the source can supply substantial currents (as these batteries can), it is vital that the supply is adequately fused, and connectors of this type should be mated with power off.

Some connectors depend entirely on a solder connection to physically retain the conductors (eg the so-called 5.5mm connectors). These connectors are often employed in conductors that could easily carry sufficient current under fault conditions to melt the solder, and the un-insulated free conductor ends could be a short circuit risk.

There are available cheap Chinese "fire proof" bags for transport and charging of LiPo batteries, but in the absence of credible tests, they may just be a marketing gimmick akin the the mobile phone RF shields!

Charging process

The charging parameters vary from cell to cell, and maintenance issues as to whether to perform a balancing charge further complicate charging operations.

Not only does the charger need to be capable of properly determining the end point of the process, the user needs to properly set charger settings on a "fits all" charger such as that shown above.

Overcharging risks explosion and fire.

 The warnings shown above require fire isolation of the batteries in normal charge, so that is somewhat less convenient than the practice we have become used to with a mobile phone for instance.

Performance

Table 1:
  Cost/energy (A$/kJ)  Energy/mass (kJ/kg) 
1500mAh 9.9V LiFe  0.158 430 
4200mAh 13.2V LiFe 0.270 380 
7200mAh 12V SLA 0.109  120 

Table 1 gives a comparison of the two batteries described in this article and a common Panasonic 7.2Ah 12V SLA battery.

Suppliers

The charger was purchased from an Australian eBay seller hobby_warehouse. Apart from the issue with the IMAX aperture obstructing the balance sockets, the supplier's service was outstanding, the goods received overnight and for very reasonable freight charge. I would deal with hobby_warehouse again.

The batteries were purchased from HobbyKing. They were an appalling supplier, they misrepresented the location of stock, failed to respond to follow up email, took weeks to actually post the goods, posted the goods in Singapore whilst representing that I was dealing with an "Australian warehouse". As noted earlier, they sell product that refers to important safety information at a web site that does not exist, or is at least not accessible. I would now avoid dealing with HobbyKing.

A footnote to dealing with HobbyKing: I have received frequent marketing emails and several attempts to unsubscribe have had no effect. The traffic was sufficient for me to block email from HobbyKing, which of course means I should not buy from them again (which is fine in the circumstance). If you deal with HobbyKing, you can expect a flood of email that cannot be stopped!

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Changes

Version Date Description
1.01 12/08/2012 Initial.
1.02    
1.03    

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