Charge Simulator

Introduction

The Satiator Charge Simulator is a web application we've produced to model the exact charging characteristics of a lithium pack under various charge scenarios. With it, you can see exactly how long it will take to charge a battery at different charging currents, both in the bulk charge regime and constant voltage as the current tapers down. You can use it to determine the charge voltage that will achieve a given % of full capacity in order to greatly improve cell cycle life while still having sufficient capacity for your range requirements. And finally, you can with a single click download the resulting optimized charge profile into a file ready to load on your Cycle Satiator.

Battery Setup

The first step in using the Charge Simulator is to select your battery settings to match the pack at hand. We have a number of preconfigured batteries (like the eZee and Allcell packs) available from the drop-down. Alternately, you can build your own custom battery by choosing your cell type and series/parallel cell count to match the capacity and voltage of your system. Your battery manufacturer or battery vendor should most definitely have this information and if they do not publish it you should insist that they make it available. It would also be documented in either the UN38.3 test report or the MSDS sheet for the pack. (As a word of caution, you should be leery of purchasing batteries from sources that do not list the exact cell type used in the pack assembly.)

While we only have a small number of Samsung, Sony, and LG cell types modeled at the moment, we expect to continue characterizing and modeling the popular cells from all the top tier manufacturers (Sony, Samsung, Panasonic, LG, Sanyo) over the year.

When the correct pack type is defined, you can further choose to connect a number of these in parallel with the ‘X 1’, ‘X 2’, ‘X 3’ ... dropdown. It's often the case that people use 2 or 3 batteries wired together for improved capacity and current handling capability, and this lets you simulate that scenario. Two 48V 10Ah eZee batteries would be equivalent to a 48V 20Ah pack etc.

Charge Profile Setup

On the "charge profile" box, you get to choose the parameters for the charge profile.

• %Charge: Here you select your desired charge level for the battery pack in terms of its nominal capacity. A 100% charge with most cells is 4.2V/cell, and at this voltage you should get the rated Amp-hours from the cell on discharge. An 80% charge would be charging to whatever voltage produces 80% of the available capacity on discharge. So a 10Ah pack with an 80% charge level should charge to exactly 8Ah instead of the full 10Ah.
• V/Cell: This is the resulting per-cell voltage required to produce the % charge level selected above. Because every lithium chemistry is unique, the exact voltage per cell needed to achieve a certain charge level will vary from one cell type to the next. This field lets you see the volts per cell that results from your selected charge level and cell type.
• Bulk Current: This is the charge current level for the Satiator. It is limited to 8A max, but in practice may be limited even lower than this with 48V batteries to stay within the 360 watt Satiator power limit. Be aware that even though you can model charging any battery at 8 amps, many ebike packs are not designed to handle this rapid of a charge current, and doing so may trip the BMS charge protection or blow the charging port fuse. In some cases you can circumvent this by doing a rapid charge through the batteries discharge port, but only if you understand the risks. Most even low current energy cells can safely be charged at a C/2 charging rate, so a 12Ah pack could be charged with a 6A charge current.
• Charge Complete: On the Satiator, there is a somewhat arbitrary point as the current gets closer to 0 amps when you decide it will say "Charge Complete". The Satiator continues to put out the full voltage and trickle current after this point, so a higher Charge Complete threshold does not terminate charging (which would be a terrible idea for cell balancing), it just shows the "charge complete" text sooner.

Understanding the Graph

The charge graph on the right updates in real-time based on the parameters you've selected for the battery and charge profile. The green plot shows the charge current with the scale on the left, red plot is the battery voltage during charging scaled by the right side axis, and vertical lines show the distinct charging regions.

• Bulk Charge: During this period, the charger is attempting to put the specified bulk current into the battery pack, while the battery voltage gradually increases to its full charge voltage in a characteristic curve that is a bit unique for each brand of cell. If the product of the bulk current and the battery voltage exceeds 360 watts, then the current will be reduced in order to maintain a constant power input at 360 watts instead.
• Constant Voltage (CV): Once the terminals of the battery reach the full voltage for the charge profile, then the charger holds constant voltage while the resulting current into the pack tapers down towards 0 amps. The duration of the CV region is shorter with cells that have low internal resistance (high 'C' rated power cells, like the Samsung 25R), and cells that have a more abrupt rise in voltage at the end of the charge curve. In some cases with high resistance batteries, the CV period can last as long as the bulk charge period, even though it is responsible for a much smaller percentage of the overall charge input.
• Charge Complete: At this point, the Satiator is simply holding a constant voltage while the current flowing into the pack has decayed to below the “charge complete” threshold and is at a neglibible value. During this region, it is possible that the battery’s BMS circuit is still actively balancing the cells, and additional trickles of current may continue to flow into the battery pack as required to maintain voltage.

The data table on the bottom summarizes the charging statistics from each of these 3 regions, so you know how long is spent in each of the charging modes and how much charge goes into each of them respectively. The watt-hours is the input energy to the battery pack, which is always higher than the watt-hours you will get out of the battery due to losses from internal resistance. However, the amp-hours in almost perfectly match the amp-hours you take out.

Downloading a Profile

The large button will download the charge profile that has been simulated into an XML file that can be opened by the Satiator Suite software and easily loaded onto your charger for use with an actual pack. This button also shows the full charge voltage and bulk current which will show up on the charge profile summary screen of the Satiator. When you click the "Download Profile" button, you will be prompted to select both a Title and Subtitle for the charge profile. Make note of where you save the file so that you can open it from inside the Satiator Suite. 

Benefits of Partial Charge

One of the key benefits of the Cycle Satiator is its ability to let you easily control the charge level of your battery. It is now well known that most lithium chemistries (with the exception of LiFePO4) can see drastic improvements in calendar and cycle life when they are not held at the nominal full charge voltage of 4.2 V/cell but are charged to a lower voltage instead. That’s how electric car manufacturers are able to 5-8 year battery warranties on cells that usually only test to ~500 cycles.

With most ebike chargers, you have no ability to set the full charge voltage and have to accept topping it up to 4.2 V/cell. This gives the most range on a charge, but if you don’t require the full capacity of your battery on most of your trips then you are unnecessarily reducing the battery life every time you charge it. In many cases that means replacing your ~$1000 lithium battery pack every 1-2 years, when with proper management it could be lasting more like 4-5 years. In fact the further from full charge you go, the more pronounced the life cycle improvements.

With the Charge Simulator we’ve made it really easy to produce profiles that will charge a battery to a given percentage of its full capacity, so you can easily create say 70%, 80%, 90%, and 100% charge curves for your pack, and the Charge Simulator will figure out the required full charge voltage for each. If you have a 20Ah battery, and typical trips only require 12Ah or less, then the 70% charge (to 14Ah) would be fitting most of the time. If you knew you needed just over 16Ah for a longer journey, then you would choose the 90% profile instead, and when you want to get full range from the battery or let the BMS balance the cells, then that is your only occasion to use the 100% profile.

Used in this manner, the Cycle Satiator will pay for itself many times over just by extending the useful life or your expensive lithium battery packs. Nevermind all the other benefits of having a compact, programmable, sealed, high power battery charger with a graphical display screen.

Partial Charge and Cell Balancing

One of the only downsides to partial charging is that many inexpensive battery management system (BMS) circuits will only do active bleed balancing of the cells when they are at or near the full charge voltage of 4.2 V/cell. This means that with partial charge profiles that don’t reach that voltage, the BMS circuit will never be able to rebalance cells if they are drifting apart. Over time you may have less available capacity from the pack as certain cells will hit the low voltage cutoff on discharge well before others.

If this is an issue it can be easily remedied by occasionally (like once every month or two) leaving the pack connected to a 100% charge cycle overnight.

Good quality programmable BMS circuits will usually attempt to balance the cells whenever they see more than a certain voltage spread between the highest and lowest cell in the group, and in that case there is no problem with partial charges. Similarly, good quality cells rarely drift out of balance in a series string, and can easily handle 100 or more cycles and maintain a perfect voltage matching even if the BMS circuit doesn’t do any active balancing. But if you aren’t sure of the makeup of your battery pack, then the protocol of occasionally giving a 100% top-up is a good bet to ensure both a long cycle life and evenly matched cell voltages.