Peter,
Are you aware of any resources on the internet that would be a good tutorial on this subject of parallelling batteries which you could share?
I don't recall seeing anything recently, most of this stuff is what you glean over the years from the battery manufacturers and hard experience.
Gates Energy were the first I recall having strong feelings about it, that was in the early 1980's with their Cyclon cells. Because they only made 2.5, 5, 7.5 and 25AH cells, a lot of people wanted higher capacity and went for parallel connection, but there were problems.
Steering diodes prevent discharge paths between cells or strings of cells in parallel, and typically you would have them both for discharge and charge current paths. Typically you would have one diode between the charging bus and each cell or string of cells, and one diode between the discharge bus and each cell or string of cells.
As the volt drop per diode is fairly constant once the diode is conducting, it is a relatively simple matter to allow for it on the charger settings and load.
Blocking diodes provide a one-way current path only and prevent back-feeding in the event of a fault.
The twin battery installation on our company website has each battery and charger feeding into a common cathode diode block (Semikron SKMD 160) with the commoned output going to the distribution baord bus.
Each charger is fused and fitted with a neutral link as well for full isolation if required, and each battery has dual fuses, so the protection is all there.
We changed the chargers and battery while the system was live, as it is part of the protection for a good part of North London, so hot working was in order.
With the twin battery installation, there is no standing load, the charger and batteries are purely standby, so no heatsinking was required for the diode block. We did mount it on a substantial frame section anyway, but the switching and closing pulses are very short, in the order of milliseconds, so the actual heat dissipation is quite small, although the current is high for that period.
There was another application we were involved in, which had hundreds of Lithium-Ion cells in series parallel for an underwater application, and that had 4 X 110V 50A chargers for the subsea vehicle. The battery supplier did all the interconnections, but it was a bit of a nightmare for them as the Li-Ion batteries also had special voltage sensing and control requirements.
Nicads can be treated in the same way, but as their self-discharge rates are higher than lead-acid, it can be more of a problem, particularly with the smaller sealed/wound cells. Vented cells are not as much of a problem but still need to be considered.
Diodes need to be rated for the maximum charge/discharge current, plus any peak loading that may be pulled through them on discharge. Thermal considerations can be critical if there is a continuous load current running through them. We work on 0.65V drop (less for Scottky Diodes) X the current to give dissipation. Heatsinking can be done relatively easily with extruded material and fans if necessary.
So, if you have 20A current going through a diode continuously, you will have approximately 13W of heat being dissipated. Get your heatsink charts and look at the thermal resistance (Deg C per Watt) and work out what you need.
When you get into the hundreds of amps then thermal considerations take a lot of sorting out, as the waste heat becomes quite serious, although the diodes are available these days up to 1000's of amps capacity.
We use Semikron / International Rectifier / Ixys product.
Dual diode modules with common cathode connection can be used with both diodes in parallel, but rate for a single diode. That gives you redundancy at almost the same cost of a single diode module which is probably more expensive than the more popular dual-diode type.
The standard Semipack type modules are industry-standard these days, and most of the heavier heatsinks come with mounting faces and slots to suit.
The main problems we see are that the smaller packs only have M5 connections for up to 100A, the next one has M6 and the largest standard module is M8. Getting a heavy cable bolted on can be a problem, so we use welding cable which is more flexible than conventional tri-rated and eases the strain on the module terminal.
Hockey-Puck modules are something else! :-))
Have to go off to the factory now,
Peter
