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Sparrow >> Modifications >> BMS (Battery Management System)

1.  Intro

BMS stands for Battery Management System. The role of a BMS is to ensure that no cell in the battery pack ever gets damaged due to anything other than old age. This is generally done by measuring voltage, current, temperature and/or charging history. Lee Hart describes the problem well:

The battery balancing problem is like an onion, with layer after layer.
1. The outermost layer is to do nothing. Just charge in series, and assume that all batteries are identical. If you do this with AGMs, they might only last one year!
2. So, you measure individual batteries, and discover that they *aren't* all identical! They all sit at different voltages. You add some method to measure individual battery voltages (Paktracker, etc.) Now you can see the low batteries, and manually charge them to fix the imbalances. The pack lasts maybe 1.5 years.
3. But this makes you want an automatic system that not only detects when battery reaches full, but will *do something* to prevent overcharging. You get something like the Rudman regulators to clamp battery voltage to 15v during charging. The batteries now last 2 years.
4. Next you learn that equal voltages does not mean equal state of charge, and that different batteries have different amphour capacities. Your range is limited by the weakest battery, and as it weakens further, your range goes down. So, you add a balancer that moves charge from stronger batteries to weaker ones, like the PowrCheq modules. Now your pack lasts 3 years.
5. Now that your batteries are lasting longer, the differences between them gets larger as they age. A weak balancing current can no longer hold them together; so you go to a higher current system like my Battery Balancer (which can charge a weak battery at up to 30 amps). Your range is now limited by the average battery's capacity, not the weakest one, and you can keep using your pack for 4 years.
6. Then you discover that old batteries (like old people) get "peculiar". Their fully charged voltage drops, their charge efficiency is worse, self-discharge is worse, etc. You need to keep separate records for each battery, and charge it according to its needs. You may also have a failure or two; rather than replace the whole pack, it's tempting to replace just the bad one and keep going. You now have significant differences between batteries, and need a balancer that you can reprogram to take these differences into account. I'm still learning how to do this on my Balancer; half the batteries in my pack are now 10 years old, and the rest are a mixed collection of replacements!
7. The next layer is to figure out what algorithms will maximize battery life, and implement them from the *beginning*. I'm guessing that you would need to program a Balancer to use different charging algorithms on the different batteries in a new pack, and compare their results.

The Sparrow, as shipped from Corbin Motors, had no BMS. This led to damage of many battery packs, as it's quite easy to drive your Sparrow in a manner which will cause the cells of the battery pack to be damaged, mostly by driving when the batteries are mostly depleted. It's also easy, given the relatively unsophisticated battery charger that came with the Sparrow, to cause the batteries to be overcharged, which damages them.

The Zivan charger that shipped with the Sparrow can be upgraded to the "V10" chip which provides a little bit of BMS-like functionality by using temperature sensors in the battery compartments.

In a specific sense, "Battery Management System" describes a spectrum of different techniques. Starting from the simple end of the spectrum:

2.  Battery Monitoring

2.1  PakTrackr

This isn't so much a battery management system as a battery monitoring system. If you are physically present and paying attention, it allows you to be your very own BMS by using your brain and a bunch of manual intervention. Mostly, it tells you when you are pushing too hard on the accelerator or when a battery is generally failing.

About $300 total? It's not clear to me which options you'd want.

2.2  CasaDelGato

Monitors voltage of all batteries in the pack, has a LED Matrix display indicating all battery voltages. Warns you if one (or more) of the batteries drops below a minimum voltage. Full kit with wiring harness for Sparrow is about $199.

3.  Battery Regulators

3.1  Home-made Zener diode regulators

Both Lee Hart Zener regs and Ed Ang regulators fall into this category. They are relatively simple devices that burn off power when a battery goes above a certain voltage. There is little to no feedback except usually some light and heat given off. Ed Ang regulators are no longer available, but they are installed already in many Sparrows. These simple dissipative regulators are good for keeping battery packs which are made up of well-matched batteries in balance. Their drawbacks are that they can only burn off a limited amount of power on each charging cycle (usually limited by the amount of heat that they can sink.) In a small battery like the Optima D34M which we use in the Sparrow, this usually means less than 1Ah of power dissipated on each charge. These regulators do nothing to keep batteries healthy while they discharge or after the charger has shut off. They generally pull a very small current from the battery while they just sit. They also generally measure only voltage, and not very exactly. Since voltage is just one variable and since temperature is important to measure, they can be very inexact.

Cost -- ~$8 per battery

3.2  Hot Juice Electric BEQ1 regs

These are a huge step up in ease of installation and durability from home-made regs. They are simple but durable. They have no feedback mechanism, so if a battery is heavily damaged and it reaches its high voltage point during the charger's bulk phase, these regulators will be asked to do more than they can possibly handle with their 1.5A shunt limit. They are probably good at keeping a new pack in good shape, though.

Cost -- $38 per battery

3.3  Manzanita Micro Mark 2 ("Rudman" MK2) regulators

These are a step up from the Lee Hart Zener regs and Ed Ang regs. They have a bigger heat sink, so they can dissipate more power. They also are adjustable, in the sense that you can dial in the high and low voltage values for your specific battery. They indicate whether a battery has gone below its low voltage point during driving with a red light which latches on until the charge cycle is complete. They indicate when a battery is being held at its high voltage point with a different LED. Most importantly, they have a feedback system that allows a battery charger (particularly those made by Manzanita Micro) to limit charge current when the regulator can no longer dissipate all the heat that it would have to if it were to hold the voltage at the high set point. Thus you can charge a pack which is very out of balance and no battery will be damaged. You might not get all the equalization you like before the charger shuts off, but no battery will be pounded on more than it can handle. These regs are temperature compensated, though not very exactly. They are more difficult to install than Hot Juice Electric regs and more fragile. If you want to install them under the hood of a Sparrow, you need to carefully "conformally coat" them such that they aren't affected by dampness, being sure that you don't coat the heat sink or the various connectors. Of course, much of the advantage of Manzanita Micro regulators comes about when you use a Manzanita Micro charger.

Cost -- $50 per battery. Another $1500 if you want a PFC charger (which has other nice features beyond just the regs)

3.4  Manzanita Micro Mark 3 (MK3) digital regulators

These are similar to the MK2's, except that they are microprocessor controlled. This allows the regulation algorithms to be a little bit more sophisticated. They can sink more energy over a charge cycle thank the MK2s, mostly because they can anticipate the need to. They are also more exactly temperature compensated and it's possible to calibrate them much more exactly for voltage measurement. Perhaps most importantly, they can communicate across a fairly noise-resistant bus (designed by Lee Hart, incidentally) which allows you to get lots of data out of them digitally. This makes it theoretically possible to use them for the same sorts of things you would want a PakTrackr for, though there is no readily available "dashboard" as there is with a PakTrackr. Instead, you can set them to record data and then get it later. Since they are actively involved in managing the batteries, one could argue that instant feedback to the user is less necessary. Note that even at this point in the spectrum, there is nothing going on which helps your batteries at any moment other than during charging. With the MK2 or MK3 regs, you could set them up to cause your motor controller to cut back when a battery drops under a certain voltage while driving, though this would require more modification of your Sparrow.

Cost -- $85 per battery. Again, you'll find that these work better with a PFC charger, so mentally add another $1500.

4.  Battery Balancers

4.1  PowerCheq

These regulators attach to two batteries at once, and move energy between them. Thus you buy twelve of them for a thirteen battery pack, with each one attached to two adjacent batteries. They move small amounts of power around all the time, trying to keep the pack in balance. As long as the batteries are relatively well balanced, or as long as you have a long time to let them sit between uses, they can be really good at keeping a pack in balance. Installation is relatively simple, except that you can't install them across the batteries which are broken up by the Big Red Button (emergency shut-off switch.) So you either need to deal with two different strings that are being separately equalized or you need to re-wire your BRB so that it doesn't break the middle of the pack, which lowers safety somewhat.

Cost - $60 per battery, only 12 needed for a stock Sparrow

4.2  BattEQ

These are very similar in nature to the PowerCheq, except that they support four batteries per module, necessitating fewer of them for the total installation.

Cost - $199 per module, only four needed for a stock Sparrow.

4.3  Lee Hart Battery Balancer

This is the most complex but most capable system that I know about. It involves a microprocessor control board, a bunch of relays and what amounts to a battery charger. It hunts through your battery pack looking for your low battery and then it charges that battery using energy from the pack as a whole. It does this while the pack is charging, while it is sitting or while you are driving. It can prop up your weak batteries with as much as thirty amps of current (roughly a hundred times what the PowerCheq can do.) This allows you to use weak batteries in your pack without having them collapse completely. I don't own one of these systems, but I have examined one closely and it's awesome to see in action. The design goal for this balancer is that it allows you to use whatever power is available from all the batteries in your pack in a way that keeps them all functioning. I believe that it hits that goal. Installation is not simple.

About $1200 total? You'd have to contact Lee for the current price.

This list above is not comprehensive. It covers only those things that I know something about and which are appropriate for a string of AGM lead-acid batteries. If you replace the Sparrow's pack with nickel or lithium, then you'll need something very different, and there are options for those too.

Jake Oshins

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Page last modified on November 01, 2009, at 12:57 PM