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To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible wit...
Answer
#4: Post edited
by
Olin Lathrop
·
2021-07-24T19:52:01Z (over 3 years ago)
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
- Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, inconvenient gate drive levels, and voltage drops, relays may start to look simple.
- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- <hr>
- <blockquote>Could I use solid state relays or would that mean having to address the same range of issues that using MOSFETs would necessitate?</blockquote>
- You could possibly use solid state relays. Their outputs are isolated from their inputs, which is certainly useful in this case. However, they usually have fairly large voltage drops relative to 12 V. SSRs are more appropriate for switching 120 V or 240 V AC where the 1 V or so drop is a small fraction of the whole. Then there is the extra complication that you really want SPDT switches with break before make.
- Mechanical relays seem like a good choice here because they have very little on-state voltage drop, can guarantee break before make in SPDT configurations, and are fully on/off in one of the states without any power applied.
The usual downsides of slow operation, power to maintain the on state, and finite lifetime switching transitions, aren't much of an issue in this case. You're only switching when the charger is connected and disconnected. It will be connected for an hour or more at a time, so a 100 ms delay at each transition is inconsequential. When the charger is connected, substantial external power is available. The extra to maintain the relays state is a tiny amount more. You're only going to connect and disconnect the charger a few 1000 times over the lifetime of the product. That's a small fraction of what any reasonable relay can do.
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
- Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, inconvenient gate drive levels, and voltage drops, relays may start to look simple.
- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- <hr>
- <blockquote>Could I use solid state relays or would that mean having to address the same range of issues that using MOSFETs would necessitate?</blockquote>
- You could possibly use solid state relays. Their outputs are isolated from their inputs, which is certainly useful in this case. However, they usually have fairly large voltage drops relative to 12 V. SSRs are more appropriate for switching 120 V or 240 V AC where the 1 V or so drop is a small fraction of the whole. Then there is the extra complication that you really want SPDT switches with break before make.
- Mechanical relays seem like a good choice here because they have very little on-state voltage drop, can guarantee break before make in SPDT configurations, and are fully on/off in one of the states without any power applied.
- The usual downsides of slow operation, power to maintain the on state, and finite lifetime switching transitions, aren't much of an issue in this case. You're only switching when the charger is connected and disconnected. It will be connected for an hour or more at a time, so a 100 ms delay at each transition is inconsequential. When the charger is connected, substantial external power is available. The extra to maintain the relays state is a tiny amount more. You're only going to connect and disconnect the charger a few 1000 times over the lifetime of the product. That's a small fraction of what any reasonable relay can do.
- <hr>
- <blockquote>Could the "maintaining the on state" issue be handled using latching type relays?</blockquote>
- Yes, but that is probably not the best tradeoff. As I said earlier, I'd wire the relay outputs so that the unpowered state is operating the device from the 48 V configuration. That means power to the relays is only required during charging.
- When charging, "wall power" is presumably available. We don't know what the power requirements are, but I'm assuming multiple amps at 12 V. Suitable relays to switch each 12 V battery might take ½ W or so. That's about 1% of the power of 4 A at 12 V, for example. The total power for a few relays should be less than a small light bulb in a typical room. I just don't see the power to run the relays when charging the batteries to be of any significance.
- On the flip side, latching relays are usually much more expensive. Since they are much less common, the selection is also more limited.
#3: Post edited
by
Olin Lathrop
·
2021-07-07T15:20:06Z (over 3 years ago)
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
- Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, inconvenient gate drive levels, and voltage drops, relays may start to look simple.
- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- <hr>
- <blockquote>Could I use solid state relays or would that mean having to address the same range of issues that using MOSFETs would necessitate?</blockquote>
- You could possibly use solid state relays. Their outputs are isolated from their inputs, which is certainly useful in this case. However, they usually have fairly large voltage drops relative to 12 V. SSRs are more appropriate for switching 120 V or 240 V AC where the 1 V or so drop is a small fraction of the whole. Then there is the extra complication that you really want SPDT switches with break before make.
Mechanical relays seem like a good choice here because they have very little on-state voltage drop, can guarantee make before break in SPDT configurations, and are fully on/off in one of the states without any power applied.- The usual downsides of slow operation, power to maintain the on state, and finite lifetime switching transitions, aren't much of an issue in this case. You're only switching when the charger is connected and disconnected. It will be connected for an hour or more at a time, so a 100 ms delay at each transition is inconsequential. When the charger is connected, substantial external power is available. The extra to maintain the relays state is a tiny amount more. You're only going to connect and disconnect the charger a few 1000 times over the lifetime of the product. That's a small fraction of what any reasonable relay can do.
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
- Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, inconvenient gate drive levels, and voltage drops, relays may start to look simple.
- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- <hr>
- <blockquote>Could I use solid state relays or would that mean having to address the same range of issues that using MOSFETs would necessitate?</blockquote>
- You could possibly use solid state relays. Their outputs are isolated from their inputs, which is certainly useful in this case. However, they usually have fairly large voltage drops relative to 12 V. SSRs are more appropriate for switching 120 V or 240 V AC where the 1 V or so drop is a small fraction of the whole. Then there is the extra complication that you really want SPDT switches with break before make.
- Mechanical relays seem like a good choice here because they have very little on-state voltage drop, can guarantee break before make in SPDT configurations, and are fully on/off in one of the states without any power applied.
- The usual downsides of slow operation, power to maintain the on state, and finite lifetime switching transitions, aren't much of an issue in this case. You're only switching when the charger is connected and disconnected. It will be connected for an hour or more at a time, so a 100 ms delay at each transition is inconsequential. When the charger is connected, substantial external power is available. The extra to maintain the relays state is a tiny amount more. You're only going to connect and disconnect the charger a few 1000 times over the lifetime of the product. That's a small fraction of what any reasonable relay can do.
#2: Post edited
by
Olin Lathrop
·
2021-07-07T11:23:29Z (over 3 years ago)
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, and voltage drops, relays may start to look simple.- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height.
- Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load.
- Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, inconvenient gate drive levels, and voltage drops, relays may start to look simple.
- I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.
- <hr>
- <blockquote>Could I use solid state relays or would that mean having to address the same range of issues that using MOSFETs would necessitate?</blockquote>
- You could possibly use solid state relays. Their outputs are isolated from their inputs, which is certainly useful in this case. However, they usually have fairly large voltage drops relative to 12 V. SSRs are more appropriate for switching 120 V or 240 V AC where the 1 V or so drop is a small fraction of the whole. Then there is the extra complication that you really want SPDT switches with break before make.
- Mechanical relays seem like a good choice here because they have very little on-state voltage drop, can guarantee make before break in SPDT configurations, and are fully on/off in one of the states without any power applied.
- The usual downsides of slow operation, power to maintain the on state, and finite lifetime switching transitions, aren't much of an issue in this case. You're only switching when the charger is connected and disconnected. It will be connected for an hour or more at a time, so a 100 ms delay at each transition is inconsequential. When the charger is connected, substantial external power is available. The extra to maintain the relays state is a tiny amount more. You're only going to connect and disconnect the charger a few 1000 times over the lifetime of the product. That's a small fraction of what any reasonable relay can do.
#1: Initial revision
by
Olin Lathrop
·
2021-07-06T21:40:15Z (over 3 years ago)
To summarize the problem, you have 12 V battery packs that you don't want to change due to certifications, but you need to power a 48 V 4 A load. The battery system also needs to be compatible with an existing charger that is meant to charge each of the 12 V packs with the negative ends tied together. Components on any new board are limited to ½ inch height. Apparently the device does not need to operate while the batteries are being charged. I would therefore consider using relays to switch the battery configuration between 48 V series connection for operating, and common ground 12 V connections while charging. The relays should be arranged so that their unpowered state is suitable for operating. When charging power is applied, the relays are energized to switch to the charging configuration, which also automatically disconnects the 48 V load. Normally I'd want to switch things with transistors. However, by the time you deal with problems due to the inherent body diode of MOSFETs, the break-before-make issues, and voltage drops, relays may start to look simple. I'm envisioning one SPDT relay at each end of each 12 V pack. Of course the two poles don't need to be in physically separate relays. A DPDT relay per pack would work, although each half would be used separately.