T-Pain Is On A Boat: Re: [Electric Boats] Battery replacement

John,

The key to batteries is to monitor them for voltage and temperature. Each battery is an electro-chemical device and is susceptible to getting hot. Even the best battery, if short circuited, will get hot.

If a battery gets hot, then either the load or the charging source needs to be turned off. I say turned off, instead of disconnected, because DC doesn't like being disconnected while current is passing through the circuit. Disconnecting DC results in an inductive spike which can damage anything on the circuit. The inductive spike is caused the collapsing of the magnetic field that surrounds a conductor when conducting current.

You have two options with combining batteries. You can either combine them at the battery level or at the string level.

If you combine at the battery level, then you can monitor the temperature of the negative busbar that combines the two batteries. If one of the batteries gets hot, then it will show up in the temp of the negative busbar, but you must monitor to closer tolerances because the temp of the busbar will be the average of the temp of the two batteries. If a battery is determined to be hot, then it should be removed and replaced.

If you combine at the string level, then each string should be independently charged and each battery monitored. The second string can be on a contactor, and if any one battery on that string gets hot, then the contactor can be disconnected. The temp monitors should give a warning so the loads or charging sources can be turned off prior to the contactor disengaging the batteries.

With any two batteries in parallel, whether at the string level, or at the battery level, and one battery goes thermal, then the other battery will drive the thermal short. You have to be very careful with regular AGM, GEL or Flooded batteries because this is a very real risk that can end up in exploding batteries and fire on board the boat.

Pure Lead Batteries do not have this fault because they don't internally short.

If you want to keep a string in reserve and function off one string only, which I don't really recommend because you get hammered by Peukerts exponent if you do this, then you should reconnect the two batteries when they are at similar resting voltages so they are at the same place in charge.

The difference between a fully charged 48 volt battery and fully discharged is only 4 or 5 volts, which is not enough to balance the two out. It takes voltage to drive current. In this case it is delta voltage. So if a fully dead battery was paralleled with a fully charged battery, and then the charger was put on, the fully charged battery would become over charged in the charge cycle which could hurt that battery, while the fully discharged battery would not be fully charged.

I put two strings of 48 volt batteries on Kapowai. Each string is independently monitored, balanced and charged. When both batteries are fully charged, I combine the two strings and go sailing. When I come back, each string is disconnected and independently charged, monitored and balanced.

James

James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103

805 455 8444

james@electroprop.com

www.electroprop.com

On Sep 14, 2018, at 9:54 PM, oak oak_box@yahoo.com [electricboats] wrote:

James,
Thank you for your thoughts on Lithium batteries.

If I go with AGM batteries instead of Lithium, and need to combine multiple batteries in parallel, can this be done safely and effectively?

I've read that for AGM batteries (and probably for other types too?) combined in parallel, that the weakest battery will discharge first, taking the other voltages down with it, but continue through charge and discharge cycles in a vicious cycle until it's dead.

Is there a good way to tie AGM batteries in parallel to avoid this?

Thanks,
John

On Friday, September 14, 2018, 11:46:15 PM CDT, James Lambden james@electroprop.com [electricboats] <electricboats@yahoogroups.com> wrote:

Hi Tom,

Lithium Batteries have higher short circuit currents so if things get out of hand with a short, there is much more current driving the short. So it is vital that each battery is fused within its enclosure. Batteries comprise one or more cells in any given enclosure. Basically current should not be allowed to flow wide open outside of each batteries enclosure.

as an example:

I have a battery that I have built in my shop. It is made up of 10 cells x 3.2 volts per cell = 32 volts. It has an accordion compression system that fully encloses the battery and provides a method to secure to the boat. There are three of these modules for 96 volts. I monitor the outside case temperature of the entire module to prevent spaghetti buildup on the battery from temp sensors. And since the enclosure is aluminum, any heat from one cell will show up elsewhere on the enclosure because aluminum is a very good conductor of heat. Each module is internally fused at 250 amps but that could be changed to 500 amps or so if necessary.

So that is how I dealt with heat monitoring and current limiting of the cells. It could be wired for 48 volts. Its a couple of years old. I suppose i should sell it soon to someone who wants it. Basically its the remnants of the Lithium R and D program that we did at Electroprop.

Each cell has to have a voltage wire attached to it going to the BMS. In my experience, I prefer Orion BMS. Those guys know what they are doing when it comes to Lithium Batteries.

The issue that I see is boats tend to be damp, especially in the working areas or engine areas. Corrosion is commonplace on boats. If you start getting corrosion where you voltage sense wires hook to your busbars, then the BMS gets faulty information and you could start overcharging the cells.

If a Lithium Battery gets fully discharged, one cell will be empty before all of the others. When current flows through an empty Lithium cell, it can fuse the cell. If you then try and charge that cell, it will go thermal. So if you have no BMS, be very careful recovering a fully discharged Lithium Battery.

The big issue with boats is that owners are not installing them the way they are professionally in other industries. Just look at the engineering that goes into a Lithium Battery Installation in a car. The batteries are heated and cooled, so environmentally protected. There are often many fuses within the battery pack if something does go wrong. None of the auto technology exists in boats. People just put them in and think of them like Lead Acid Batteries. Lead Acid is very forgiving in comparison to Lithium.

I do think that when Lithium Batteries are properly engineered into a new boat, that would most likely be ok and very safe. But random placement of Lithium on a boat is not giving the respect that a Lithium battery requires. They are fantastic batteries, that is no doubt. But if you have a fire on a boat, you are in a whole lot more trouble, than if you have a fire in a car, where you can just pull over and get out. Lithium batteries are required to be shipped as hazardous material. Why do you want to put hazardous material on a sailing boat? I can understand a tender, but surely not the mothership.

I admit there are exceptions to my broad observations and it can be done. But there is a lot more engineering required to put a Lithium Battery aboard a boat than there is lead acid. And the cost is through the roof if you do it right.

I will stick with Lead Acid until a battery can solve these issues at a respectable cost. I don't mind the extra weight of a lead battery on my Catalina 30. I find it makes the boat much more predictable, less likely to be buffeted by oncoming waves, and more stable overall. Plus I just don't have to worry about them at all.

Kapowai has two strings of Northstar 155 amp hour batteries for a total of 15 KWH's of battery storage. Kapowai is a Catalina 30 powered by a 450 watt solar panel capable of tilting and turning to focus on the sun. Kapowai is as close as you can get to a perpetual motion machine, in practical terms, because she is fully energy self sufficient and can travel in perpetuity, as long as there is either sun or wind.

James

James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103

805 455 8444

james@electroprop.com

www.electroprop.com

On Sep 13, 2018, at 1:14 PM, boat_works@yahoo.com [electricboats] wrote:

Hi James,

Would you please elaborate on your statement:

"I personally would not go the LIPO route as there are issues over time with LIPO batteries".

Thanks!
-Tom

__._,_.___

Posted by: James Lambden <james@electroprop.com>

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