El  -­‐  Technology  

 Review  of  SRAM  E  Matic  battery  pack    (10Ah  version)  

commissioned  by  Cycle  service  Nordic  Aps  Odense,  Denmark  

Engineering  review  ver1.1  

El-Technology Engineering review

Table  of  contents  

Introduction: .............................................................................................................................. 3  Background............................................................................................................................... 3  Mechanical................................................................................................................................ 4  Front end cap............................................................................................................................ 4  Rear end cap ............................................................................................................................ 5  Main body extrusion.................................................................................................................. 5  Electrical and battery ................................................................................................................ 5  Testing ...................................................................................................................................... 7  Conclusions .............................................................................................................................. 9  

El-Technology Engineering review

Introduction  

This document reports the findings of an engineering review of an SRAM, E-matic battery. The purpose of the review was to determine the overall quality and fitness for purpose of the battery pack. The report is divided into four sections:

• Background • Mechanical • Battery and electrical • Testing

The work was done in May 2014 at the premises El-Technology.

Figure 1 Front end cap is item 1, rear end cap item 2 and main body extrusion is item 3

Background  

SRAM is a well known supplier of high end mountain and road bike components. The company grew out of the American company Gripshift in the 90s and grew by acquiring other brands such as Sachs, Rockshox and Avid. It is to be expected that any product carrying their

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El-Technology Engineering review brand would be high quality. The package received from LiTHIUM BALANCE A/S included a battery pack, charger and dealer test tool with the associated cabling. The dealer test tool includes a built in load resistor to allow charge and discharge tests on the battery pack.

Mechanical  

Figure 2 Battery pack dismantled

The battery pack is made of an anodised, aluminium extrusion with two injection moulded plastic end caps. The front end cap (see Fig 1) contains two fuses and a multiport connector for the test tool. The rear end cap contains the on/off switch, charge status monitor and the charging socket.

Front  end  cap  Comments: The mounted connector is of good quality containing both power and data connections and is rigidly mounted on the end cap with two screws. The fuse holders are also held in place with screws. The end cap has a rubber seal between it and the main body extrusion of the battery pack. Connectors and fuses are both protected by protruding flanges that ensure that they are not the first part of the battery pack to hit the ground if it is dropped. Four socket head bolts are used to secure it to the main body extrusion. Conclusions: This item is well designed and made and fully fit for purpose providing adequate sealing and protection for the battery pack connectors present and the cells/electronics inside.

El-Technology Engineering review

Figure 3 Inside of Rear end cap showing shock protection pads (1) and built in buzzer (2)

Rear  end  cap  Comments: There is a nicely finished anodised aluminium carrying handle integrated into the end cap. The illuminated on/off switch and the charging socket are placed on either side of the end cap with screws holding the carrying handle in place. There is a moulded rubber seal running around the end cap where it joins to the main extrusion body. All screws appear to have some basic locking compound applied to stop them loosening. There is also a piezo electric buzzer mounted with two screws inside. Two robust rubber shock protection pads are positioned inside to ensure that the battery has some shock protection if the pack is dropped on the end cap. The end cap is mounted using four flush mounted socket head bolts. Conclusions: This item is thoughtfully designed, well made and fully fit for purpose. I noticed that the two mounting screws next to the on/off switch were too short and had been previously removed; as such they are no longer firmly engaged.

Main  body  extrusion  Comments: This is a robust aluminium extrusion with a 2mm wall thickness. It has a high quality anodised finish and tapped holes for the bolts that hold on the end caps. Conclusions: This is overly heavy and robust in design adding unwanted weight, it is, however, well made and fit for purpose. The battery has padded strips running down both sides and along the top to ensure a snug fit into the extrusion to provide impact protection in case of accidental dropping of the battery.

Electrical  and  battery  

The battery: The battery appears to be made up of made up 52 x 18650 Li Ion battery cells

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El-Technology Engineering review similar to those commonly found in computer laptops. These cells are, according to the documentation on the battery pack, made by Samsung SDI and are listed on the Samsung home page as being designed specifically for E-bikes. According to SRAM the cells used are ICR 18650 22P cells. They have a capacity of 2150mAh and are connected with a 5p10s configuration. The chemistry is NCM which provides the best all round balance between energy density, power performance and cycle life. The cells are used to 90% depth of discharge* (DoD) which is a bit higher than I would recommend and will definitely shorten cycle life. For industrial and automotive applications you would typically use 80% DoD. All battery electrodes are isolated from the battery main body extrusion and each other using a dielectric strip, which is brittle, but adequate for this application. The reality is that the battery could be made lighter and to last longer by using LFP pouch cells and a fully plastic body, but nobody does this - so what you have here is as good as it gets for an E-bike battery. The quoted motor performance is 250Watts which works out at around 6-7A discharge current. This is always less than 1C and the cells are rated at 2C continuous discharge, meaning that the battery cells are being used within their comfort zone. Battery management system (BMS): There is a fully specified, microprocessor controlled, battery management system with built-in power electronics that can disconnect the battery to prevent over discharge. The switching mosfets (semi-conductor switches for disconnecting the pack) are rated at 200W which makes them correctly dimensioned if used in parallel. I was unable to check these functions. There are also balancing resistors (100 Ohm giving a 40mA balancing current) which will keep the cells all operating at similar individual voltages and maximise the pack capacity. In addition to the BMS there is a battery state of charge (SoC) display and a piezo-electric warning buzzer both of which are very nice features to include in an E-bike battery. There is extensive use of dedicated connectors with locking tabs and only some of the power bus wires making do with cheap spade type connectors crimped onto the cables. This is important as unreliable or poor connections are often a source of failure in battery packs. It is difficult to see the extent of the thermal management features installed, other than the two temperature sensors that appear to be taped to battery cells. Whether the BMS is capable of preventing charging when the pack is too cold or too hot should be checked with SRAM. If this feature is not present, then it needs to be added as charging batteries at low temperatures irreversibly damages the cells and may lead to internal short circuits within the cells. *Depth of discharge is the percentage of the total battery capacity, that is used in each full operational cycle

El-Technology Engineering review

Figure 4 Inside of front end cap showing BMS - bleeding resistor array item 1; main processor item 2 and item 3 is the mosfet.

Testing  

A number of partial charge and discharge tests were made using the accompanying Dealer test tool and PC software with mixed results. Charge and discharge tests showed that the battery and the battery cells performed faultlessly; however, the testing tool was prone to overheating and would go on strike regularly.

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El-Technology Engineering review

Figure 5 Initial cell voltages before discharge test The initial measurement shows the cells to all be within 7mV of each other which is exceptionally well balanced, typically you attempt to balance your cells to within 50mV of each other. After discharge, all of the cells were still within 8mV of each other which also indicates well matched cells and a properly balanced pack. A multi-meter was used to check pack voltage and that gave 41.2 volts which corresponds well with the 41.3volts shown in the PC software and also suggests that there is a separate pack voltage measurement circuit probably within the Dealer test tool as the sum of the individual cells voltages is 41.4volts. Other features such as cell resistance are calculated in the PC software and point towards a capable BMS system. Unfortunately, the main microprocessor has been covered with epoxy making it impossible to identify it, however, this is clearly a decent sized microprocessor - you can see from the pin count – which is consistent with my previous comments.

El-Technology Engineering review

Figure 6 Cell voltages after discharge

Conclusions  

This is a thoughtfully designed and well built battery pack incorporating good features and good quality components. The battery has been designed to be capable of withstanding the battery being dropped onto a hard surface as well as being able to handle water splashes. The nature of the cells and the design of the main body of the battery pack provide sufficient protection against catastrophic cell failure which would ruin the pack but not present any danger to the user. The only concern or disadvantage of this solution is the weight, this is a heavy battery and weight has historically been the enemy of bicycles. El-Technology June 2014